Facebook is a social networking service launched in February 2004, owned and operated by Facebook, Inc. As of June 2012, Facebook has over 955 million active users, more than half of them using Facebook on a mobile device. Users must register before using the site, after which they may create a personal profile, add other users as friends, and exchange messages, including automatic notifications when they update their profile. Additionally, users may join common-interest user groups, organized by workplace, school or college, or other characteristics, and categorize their friends into lists such as "People From Work" or "Close Friends". Facebook was founded by Mark Zuckerberg with his college roommates and fellow students Eduardo Saverin, Andrew McCollum, Dustin Moskovitz and Chris Hughes. The website's membership was initially limited by the founders to Harvard students, but was expanded to other colleges in the Boston area, the Ivy League, and Stanford University. It gradually added support for students at various other universities before opening to high school students, and eventually to anyone aged 13 and over. However, according to a May 2011 Consumer Reports survey, there are 7.5 million children under 13 with accounts and 5 million under 10, violating the site's terms of service. A January 2009 Compete.com study ranked Facebook as the most used social networking service by worldwide monthly active users. Entertainment Weekly included the site on its end-of-the-decade "best-of" list, saying, "How on earth did we stalk our exes, remember our co-workers' birthdays, bug our friends, and play a rousing game of Scrabulous before Facebook?" Critics, such as Facebook Detox, state that Facebook has turned into a national obsession in the United States, resulting in vast amounts of time lost and encouraging narcissism. Quantcast estimates Facebook has 138.9 million monthly unique U.S. visitors in May 2011. According to Social Media Today, in April 2010 an estimated 41.6% of the U.S. population had a Facebook account. Nevertheless, Facebook's market growth started to stall in some regions, with the site losing 7 million active users in the United States and Canada in May 2011.In September 2012, Zuckerberg speaking about the drop in his company's market value, described the decline as "disappointing" - the value of Facebook being almost half the $38 debut price in May 2012. The name of the service stems from the colloquial name for the book given to students at the start of the academic year by some university administrations in the United States to help students get to know each other. Facebook allows any users who declare themselves to be at least 13 years old to become registered users of the site. Mark Zuckerberg wrote Facemash, the predecessor to Facebook, on October 28, 2003, while attending Harvard as a sophomore. According to The Harvard Crimson, the site was comparable to Hot or Not, and "used photos compiled from the online facebooks of nine houses, placing two next to each other at a time and asking users to choose the 'hotter' person" Mark Zuckerberg co-created Facebook in his Harvard dorm room. To accomplish this, Zuckerberg hacked into the protected areas of Harvard's computer network and copied the houses' private dormitory ID images. Harvard at that time did not have a student "facebook" (a directory with photos and basic information), though individual houses had been issuing their own paper facebooks since the mid-1980s. Facemash attracted 450 visitors and 22,000 photo-views in its first four hours online. The site was quickly forwarded to several campus group list-servers, but was shut down a few days later by the Harvard administration. Zuckerberg was charged by the administration with breach of security, violating copyrights, and violating individual privacy, and faced expulsion. Ultimately, the charges were dropped. Zuckerberg expanded on this initial project that semester by creating a social study tool ahead of an art history final, by uploading 500 Augustan images to a website, with one image per page along with a comment section. He opened the site up to his classmates, and people started sharing their notes. The following semester, Zuckerberg began writing code for a new website in January 2004. He was inspired, he said, by an editorial in The Harvard Crimson about the Facemash incident. On February 4, 2004, Zuckerberg launched "Thefacebook", originally located at thefacebook.com. Six days after the site launched, three Harvard seniors, Cameron Winklevoss, Tyler Winklevoss, and Divya Narendra, accused Zuckerberg of intentionally misleading them into believing he would help them build a social network called HarvardConnection.com, while he was instead using their ideas to build a competing product. The three complained to the Harvard Crimson, and the newspaper began an investigation. The three later filed a lawsuit against Zuckerberg, subsequently settling. Membership was initially restricted to students of Harvard College, and within the first month, more than half the undergraduate population at Harvard was registered on the service. Eduardo Saverin (business aspects), Dustin Moskovitz (programmer), Andrew McCollum (graphic artist), and Chris Hughes soon joined Zuckerberg to help promote the website. In March 2004, Facebook expanded to Stanford, Columbia, and Yale. It soon opened to the other Ivy League schools, Boston University, New York University, MIT, and gradually most universities in Canada and the United States. Facebook was incorporated in mid-2004, and the entrepreneur Sean Parker, who had been informally advising Zuckerberg, became the company's president. In June 2004, Facebook moved its base of operations to Palo Alto, California. It received its first investment later that month from PayPal co-founder Peter Thiel. The company dropped The from its name after purchasing the domain name facebook.com in 2005 for $200,000. Total active users[N 1] Users Date Days later Monthly growth[N 2] (in millions) August 26, 2008 100 1,665 178.38%  April 8, 2009 200 225 13.33%  September 15, 2009 300 160 9.38%  February 5, 2010 400 143 6.99%  July 21, 2010 500 166 4.52% [N 3] January 5, 2011 600 168 3.57%  May 30, 2011 700 145 3.45%  September 22, 2011 800 115 3.73%  April 24, 2012 900 215 1.74% Facebook launched a high-school version in September 2005, which Zuckerberg called the next logical step. At that time, high-school networks required an invitation to join. Facebook later expanded membership eligibility to employees of several companies, including Apple Inc. and Microsoft. Facebook was then opened on September 26, 2006, to everyone of age 13 and older with a valid email address. On October 24, 2007, Microsoft announced that it had purchased a 1.6% share of Facebook for $240 million, giving Facebook a total implied value of around $15 billion. Microsoft's purchase included rights to place international ads on Facebook. In October 2008, Facebook announced that it would set up its international headquarters in Dublin, Ireland. In September 2009, Facebook said that it had turned cash-flow positive for the first time. In November 2010, based on SecondMarket Inc., an exchange for shares of privately held companies, Facebook's value was $41 billion (slightly surpassing eBay's) and it became the third largest U.S. Web company after Google and Amazon. Traffic to Facebook increased steadily after 2009. More people visited Facebook than Google for the week ending March 13, 2010. In March 2011 it was reported that Facebook removes approximately 20,000 profiles from the site every day for various infractions, including spam, inappropriate content and underage use, as part of its efforts to boost cyber security. In early 2011, Facebook announced plans to move to its new headquarters, the former Sun Microsystems campus in Menlo Park, California. Release of statistics by DoubleClick showed that Facebook reached one trillion pageviews in the month of June 2011, making it the most visited website in the world. It should, however, be noted that Google and some of its selected websites are not counted in the DoubleClick rankings. According to the Nielsen Media Research study, released in December 2011, Facebook is the second most accessed website in the US. In March 2012, Facebook announced App Center, an online mobile store which sells applications that connect to Facebook. The store will be available to iPhone, Android and mobile web users. In March 2012, members of the LGBTI community asked Facebook to add a "other", "third gender" or "intersex" tab in the gender option which contains only male and female. Facebook refused and said that individuals can "opt out" of showing their gender. Facebook, Inc. held an initial public offering on May 17, 2012, negotiating a share price of $38 apiece, valuing the company at $104 billion, the largest valuation to date for a newly listed public company. On August 23rd, 2012 Facebook released the much anticipated update to its iOS app, version 5.0. The app, which did not receive positive sentiments from its users, was rebuilt from the ground up; the app no longer uses page views which made it slow in the past but now utilizes code that uses native elements of iOS. The result is a much smoother, faster, and easier to use app.  User Profile Users can create profiles with photos, lists of personal interests, contact information, and other personal information. Users can communicate with friends and other users through private or public messages and a chat feature. They can also create and join interest groups and "like pages" (called "fan pages" until April 19, 2010), some of which are maintained by organizations as a means of advertising. A 2012 Pew Internet and American Life study identified that between 20–30% of Facebook users are "power users" who frequently link, poke, post and tag themselves and others. Privacy Settings To allay concerns about privacy, Facebook enables users to choose their own privacy settings and choose who can see specific parts of their profile. The website is free to users, and generates revenue from advertising, such as banner ads. Facebook requires a user's name and profile picture (if applicable) to be accessible by everyone. Users can control who sees other information they have shared, as well as who can find them in searches, through their privacy settings. Comparison with Myspace The media often compares Facebook to MySpace, but one significant difference between the two Web sites is the level of customization. Another difference is Facebook's requirement that users give their true identity, a demand that MySpace does not make. MySpace allows users to decorate their profiles using HTML and Cascading Style Sheets (CSS), while Facebook allows only plain text. Facebook has a number of features with which users may interact. They include the Wall, a space on every user's profile page that allows friends to post messages for the user to see; Pokes, which allows users to send a virtual "poke" to each other (a notification then tells a user that they have been poked); Photos, where users can upload albums and photos; and Status, which allows users to inform their friends of their whereabouts and actions. Depending on privacy settings, anyone who can see a user's profile can also view that user's Wall. In July 2007, Facebook began allowing users to post attachments to the Wall, whereas the Wall was previously limited to textual content only. News Feed On September 6, 2006, a News Feed was announced, which appears on every user's homepage and highlights information including profile changes, upcoming events, and birthdays of the user's friends. This enabled spammers and other users to manipulate these features by creating illegitimate events or posting fake birthdays to attract attention to their profile or cause. Initially, the News Feed caused dissatisfaction among Facebook users; some complained it was too cluttered and full of undesired information, others were concerned that it made it too easy for others to track individual activities (such as relationship status changes, events, and conversations with other users). In response, Zuckerberg issued an apology for the site's failure to include appropriate customizable privacy features. Since then, users have been able to control what types of information are shared automatically with friends. Users are now able to prevent user-set categories of friends from seeing updates about certain types of activities, including profile changes, Wall posts, and newly added friends. On February 23, 2010, Facebook was granted a patent on certain aspects of its News Feed. The patent covers News Feeds in which links are provided so that one user can participate in the same activity of another user. The patent may encourage Facebook to pursue action against websites that violate its patent, which may potentially include websites such as Twitter. One of the most popular applications on Facebook is the Photos application, where users can upload albums and photos. Facebook allows users to upload an unlimited number of photos, compared with other image hosting services such as Photobucket and Flickr, which apply limits to the number of photos that a user is allowed to upload. During the first years, Facebook users were limited to 60 photos per album. As of May 2009, this limit has been increased to 200 photos per album. Privacy settings can be set for individual albums, limiting the groups of users that can see an album. For example, the privacy of an album can be set so that only the user's friends can see the album, while the privacy of another album can be set so that all Facebook users can see it. Another feature of the Photos application is the ability to "tag", or label, users in a photo. For instance, if a photo contains a user's friend, then the user can tag the friend in the photo. This sends a notification to the friend that they have been tagged, and provides them a link to see the photo. On 7 June 2012, Facebook launched its App Center to its users. It will help the users in finding games and other applications with ease. Since the launch of the App Center, Facebook has seen 150M monthly users with 2.4 times the installation of apps.  Facebook Notes Facebook Notes was introduced on August 22, 2006, a blogging feature that allowed tags and embeddable images. Users were later able to import blogs from Xanga, LiveJournal, Blogger, and other blogging services. During the week of April 7, 2008, Facebook released a Comet-based instant messaging application called "Chat" to several networks, which allows users to communicate with friends and is similar in functionality to desktop-based instant messengers. Facebook launched Gifts on February 8, 2007, which allows users to send virtual gifts to their friends that appear on the recipient's profile. Gifts cost $1.00 each to purchase, and a personalized message can be attached to each gift. On May 14, 2007, Facebook launched Marketplace, which lets users post free classified ads. Marketplace has been compared to Craigslist by CNET, which points out that the major difference between the two is that listings posted by a user on Marketplace are seen only by users in the same network as that user, whereas listings posted on Craigslist can be seen by anyone. On July 20, 2008, Facebook introduced "Facebook Beta", a significant redesign of its user interface on selected networks. The Mini-Feed and Wall were consolidated, profiles were separated into tabbed sections, and an effort was made to create a "cleaner" look. After initially giving users a choice to switch, Facebook began migrating all users to the new version starting in September 2008. On December 11, 2008, it was announced that Facebook was testing a simpler signup process. Facebook Username On June 13, 2009, Facebook introduced a "Usernames" feature, whereby pages can be linked with simpler URLs such as http://www.facebook.com/facebook instead of  http://www.facebook.com/profile.php?id=20531316728. Many new smartphones offer access to Facebook services through either their Web browsers or applications. An official Facebook application is available for the operating systems Android, iOS, and webOS. Nokia and Research In Motion both provide Facebook applications for their own mobile devices. More than 425 million active users access Facebook through mobile devices across 200 mobile operators in 60 countries. Facebook Messages On November 15, 2010, Facebook announced a new "Facebook Messages" service. In a media event that day, CEO Mark Zuckerberg said, "It's true that people will be able to have an @facebook.com email addresses, but it's not email". The launch of such a feature had been anticipated for some time before the announcement, with some calling it a "Gmail killer". The system, to be available to all of the website's users, combines text messaging, instant messaging, emails, and regular messages, and will include privacy settings similar to those of other Facebook services. Codenamed "Project Titan", Facebook Messages took 15 months to develop. In February 2011, Facebook began to use the hCalendar microformat to mark up events, and the hCard microformat for the events' venues, enabling the extraction of details to users' own calendar or mapping applications. Voice Calls Since April 2011 Facebook users have had the ability to make live voice calls via Facebook Chat, allowing users to chat with others from all over the world. This feature, which is provided free through T-Mobile's new Bobsled service, lets the user add voice to the current Facebook Chat as well as leave voice messages on Facebook. Video Calling On July 6, 2011, Facebook launched its video calling services using Skype as its technology partner. It allows one to one calling using a Skype Rest API. Facebook Subscribe On September 14, 2011, Facebook launched a Subscribe button. The feature allows for users to follow public updates, and these are the people most often broadcasting their ideas. There were major modifications that the site released on September 22, 2011. As reported by TechCrunch on February 15, 2012, Facebook is introducing ‘Verified Account’ concept like that of Twitter & Google+. Though as of March 3, 2012, verified accounts don’t get any badges or denotations, but such accounts will get more priority in ‘Subscription Suggestions’ of Facebook. On March 6, 2012, Facebook officially launched Messenger for Windows, which gives users of Windows 7 access to some Facebook services without using a web browser. Privacy According to comScore, an internet marketing research company, Facebook collects as much data from its visitors as Google and Microsoft, but considerably less than Yahoo!. In 2010, the security team began expanding its efforts to reduce the risks to users' privacy, but privacy concerns remain. On November 6, 2007, Facebook launched Facebook Beacon, which was an ultimately failed attempt to advertise to friends of users using the knowledge of what purchases friends made. As of March 2012, Facebook's usage of its user data is under close scrutiny. FTC settlement On November 29, 2011, Facebook agreed to settle US Federal Trade Commission charges that it deceived consumers by failing to keep privacy promises. Technical aspects Facebook is built in PHP which is compiled with HipHop for PHP, a source code transformer built by Facebook engineers that turns PHP into C++. The deployment of HipHop reportedly reduced average CPU consumption on Facebook servers by 50%. Facebook is developed as one monolithic application. According to an interview in 2012 with Chuck Rossi, a build engineer at Facebook, Facebook compiles into a 1.5 GB binary blob which is then distributed to the servers using a custom BitTorrent-based release system. Rossi stated that it takes approximately 15 minutes to build and 15 minutes to release to the servers. The build and release process is zero downtime and new changes to Facebook are rolled out daily. Like button Further information: Like button The Like button is one of Facebook's social plug-ins. It was launched on April 21, 2010. Dislike button Recently, the 'dislike button' for Facebook has become available to users as a file download for web browsers on numerous sites (such as the Mozilla.org website) which they can use as an add-on on their web browsers to 'dislike' people's posts and comments.