Facebook currently allows up to 5000 friends. Assume that birthdays are uniformly distributed among 365 days. It is well-known that for a user with 23 friends there is a 50% chance of having to write ”Happy Birthday” on at least two walls in the same day.
How many facebook friends should one acquire to have a 50% chance of at least 10 birthdays falling on the same day? With 3286 friends this is guaranteed to happen, but one should expect that 50% probability will be reached with a substantially smaller number.
[Update] In 1989 Diaconis and Mosteller found an approximate solution that gives an approximate number of people required to have at least 50% probability of of at least birthdays on the same day. The formula can be found on MathWorld and its output is the OEIS sequence A050255. For comparison, Diaconis and Mosteller quote exact values found by Bruce Levin. The exact solution is the sequence A014088, which begins thus:
1, 23, 88, 187, 313, 460, 623, 798, 985, 1181, 1385, 1596, 1813, 2035, 2263
So, with 1181 Facebook friends there is a 50% chance of having to write ”Happy Birthday” at least ten times in the same day.