That […] online social networks will increase the size of human social groups is an obvious hypothesis, given that they reduce a lot of the friction and cost involved in keeping in touch with other people. […]
Primatologists call at least some of the things that happen on social networks “grooming“. In the wild, grooming is time-consuming and here computerisation certainly helps. But keeping track of who to groom—and why—demands quite a bit of mental computation. You need to remember who is allied with, hostile to, or lusts after whom, and act accordingly. Several years ago, therefore, Robin Dunbar, an anthropologist who now works at Oxford University, concluded that the cognitive power of the brain limits the size of the social network that an individual of any given species can develop.
Two items of note: Facebook has an “in-house sociologist”; and this man, Dr Cameron Marlow, reveals that the average number of friends correlates pretty closely to Dunbar’s number.
via Mind Hacks