User base growth model for viral web marketing 
THis is a model for predicting the growth of the user base of a given application or service, provided that there is a mechanism and motivation for users to invite their friends to join.
The churn table is used to estimate the rate of user desertion. to make proper use of it, you have to enter your assumed chun numbers in cells N4 through T4, then go to the "churn graph" sheet, copy the numbers for A, B from the equations on the graph (Y = A X ^ B) into cells C24 and D24, and this will update the churn sums on the "PRojection" sheet.
Assumptions: Invitations: Month 1 2 3 User acq. cost: $1.00 Invites per user 0.6 0.25 0 Month New acquired users New Invited Users invited by 1-m old users invited by 2-m old users invited by 3-m old users Total New Users this month Aug-07 0 Sep-07 0 Oct-07 0 Nov-07 0 0 0 0 Dec-07 10,000 0 0 0 10,000 Jan-08 10,000 4,800 0 0 14,800 Feb-08 11,004 2,000 0 13,004 Mar-08 15,314 4,585 0 19,899 Apr-08 22,507 6,381 0 28,888 May-08 33,118 9,378 0 42,496 Jun-08 48,797 13,799 0 62,596 Jul-08 71,990 20,332 0 92,322 Aug-08 106,357 29,996 0 136,352 Sep-08 157,120 44,315 0 201,436 Oct-08 232,037 65,467 0 297,504 Nov-08 342,713 96,682 0 439,395 Dec-08 506,186 142,797 0 648,983 Jan-09 747,647 210,911 0 958,558 Feb-09 1,104,303 311,520 0 1,415,822 Mar-09 1,631,106 460,126 0 2,091,232 Apr-09 2,409,229 679,627 0 3,088,857 May-09 3,558,569 1,003,846 0 4,562,414 Jun-09 5,256,217 1,482,737 0 6,738,954 Jul-09 7,762,087 2,190,090 0 9,952,177 Aug-09 11,462,649 3,234,203 0 14,696,852 Sep-09 16,928,774 4,776,104 0 21,704,878 Oct-09 25,001,274 7,053,656 0 32,054,930 Nov-09 36,923,312 10,417,197 0 47,340,509 Dec-09 54,530,499 15,384,713 0 69,915,213Churn: % remaining at end of month Month 0 1 2 3 4 5 % left: 100.0% 80.0% 65.0% 55.0% 50.0% 46.0% Sum Total Users 0000 8,000 8,000 18,340 11,840 6,500 25,523 10,403 9,620 5,500 37,512 15,919 8,453 8,140 5,000 55,197 23,110 12,934 7,152 7,400 4,600 81,328 33,997 18,777 10,944 6,502 6,808 119,983 50,077 27,622 15,888 9,949 5,982 177,261 73,857 40,688 23,373 14,444 9,154 261,867 109,082 60,009 34,428 21,248 13,288 386,729 161,148 88,629 50,777 31,298 19,548 571,189 238,003 130,933 74,994 46,161 28,794 843,643 351,516 193,378 110,790 68,176 42,468 1,246,079 519,186 285,607 163,627 100,718 62,722 1,840,504 766,847 421,839 241,667 148,752 92,660 2,718,510 1,132,658 623,063 356,941 219,698 136,852 4,015,382 1,672,986 920,285 527,207 324,491 202,122 5,930,948 2,471,086 1,359,301 778,702 479,279 298,532 8,760,361 3,649,932 2,007,757 1,150,178 707,911 440,937 12,936,811 5,391,163 2,965,569 1,698,871 1,045,616 651,278 19,104,415 7,961,742 4,380,320 2,509,328 1,544,428 961,967 28,214,623 11,757,482 6,468,915 3,706,425 2,281,207 1,420,874 41,668,790 17,363,902 9,552,954 5,473,697 3,369,477 2,098,711 61,538,853 25,643,944 14,108,170 8,083,269 4,976,088 3,099,919 90,884,166 37,872,407 20,835,704 11,937,683 7,348,426 4,578,001 134,223,124 55,932,170 30,771,331 17,630,211 10,852,439 6,760,5526 7 8 9 10 11 12 13 43.0% 41.0% 40.9% 38.8% 37.0% 35.4% 34.1% 32.9% 4,300 6,364 4,100 5,592 6,068 4,086 8,557 5,332 6,047 3,876 12,422 8,159 5,314 5,737 3,697 18,273 11,844 8,131 5,040 5,472 3,543 26,916 17,423 11,804 7,713 4,808 5,243 3,407 39,698 25,664 17,364 11,197 7,357 4,607 5,043 3,287 58,632 37,852 25,578 16,472 10,681 7,050 4,431 4,865 86,617 55,904 37,724 24,263 15,712 10,234 6,780 4,275 127,927 82,589 55,716 35,785 23,144 15,055 9,843 6,541 188,940 121,977 82,309 52,851 34,134 22,176 14,480 9,496 279,063 180,152 121,564 78,078 50,414 32,707 21,328 13,969 412,180 266,083 179,543 115,315 74,477 48,306 31,457 20,577 608,804 393,009 265,184 170,313 109,997 71,364 46,459 30,348 899,230 580,487 391,681 251,551 162,459 105,398 68,635 44,822 1,328,208 857,405 578,525 371,544 239,950 155,667 101,368 66,216 1,961,838 1,266,431 854,507 548,783 354,411 229,918 149,714 97,796 2,897,750 1,870,590 1,262,151 810,577 523,476 339,593 221,126 144,439 4,279,436 2,762,971 1,864,267 1,197,264 773,197 501,590 326,607 213,33614 15 16 17 18 31.8% 30.8% 30.0% 29.1% 28.4% 3,180 4,706 3,083 4,135 4,563 2,995 6,328 4,009 4,433 2,915 9,186 6,135 3,895 4,314 2,841 13,513 8,906 5,960 3,791 4,205 19,905 13,102 8,652 5,800 3,695 29,357 19,299 12,728 8,421 5,654 43,358 28,463 18,749 12,387 8,208 64,054 42,038 27,652 18,246 12,074 94,602 62,104 40,840 26,911 17,785 139,722 91,722 60,333 39,745 26,230Churn Graph Trend params A 1.0375 B -0.4481 0.0% 20.0% 40.0% 60.0% 80.0% 100.0% 120.0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14y = 1.0375x-0.4481 R2 = 0.9929 13 14 15 16 17 18 19 20 21 22 23 24 25 Series1 Power (Series1)Power (Series1)total users at end of month Total UA cost per month Accumulated UA Cost Total Accumulated Effective UAC #REF! $10,000 $10,000 #REF! #REF! $10,000 $20,000 #REF! $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 $0 $20,000 $0.00 #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF! #REF!