I first ran across Dunbar’s number in The Tipping Point. Robin Dunbar estimates that humans can maintain, on average, a group size of 150 people. Beyond that people are more like strangers. The Economist interviewed Cameron Marlow from Facebook, who has analyzed how people socialize on Facebook. The numbers are better summarized here. The key point is that people appear to have a close relationship (measured by regular communication) with only 5% of their overall group. The average number of friends on Facebook is 120 (though a big range), but people regularly communicate with only around 6 of those people (women have a few more close friends than men). I don’t think you can equate Facebook friends with Dunbar’s number because it’s too easy to add random people and never think about them again. But it’s interesting that the average is around the same.