Dunbar’s number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person. Proponents assert that numbers larger than this generally require more restricted rules, laws, and enforced norms to maintain a stable, cohesive group. No precise value has been proposed for Dunbar’s number, but a commonly cited approximation is 150.

According to Cameron Marlow, Facebook’s “in-house sociologist,” that number is four if you are male and six if you are female. As the Economist reports this morning, Marlow’s research indicates that the average Facebook user has a network of about 120 friends, but only has two-way conversations with a very small subset of these ‘friends.’ Interestingly, even for those users who have a far larger number of friends (500+), those numbers barely grow (ten for men and sixteen for women).