Killworth, et al. found using this technique and others that the number of people a person will know in their lifetime ranges somewhere between 300 and 3000[1].
On Facebook, the average number of friends that a person has is currently 120[2]. Given that Facebook has only been around for 5 years, that not everyone uses it, and that the not every acquaintance has found each other, this number seems reasonable for an average user.