How many friends can you have?

For casual friends, the number is about 150, the so-called Dunbar number.

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. Extrapolating from the brain sizes and social networks of apes, Dr Dunbar suggested that the size of the human brain allows stable networks of about 148. Rounded to 150, this has become famous as “the Dunbar number”.

So they test out the theory with social networking website Facebook, and it turns out to be pretty close: on average a user has 120 friends listed in Facebook. The number of active friends, indicated by leaving comments on friends' postings, is about 7 for men and 10 for women. Close friends -- defined by two-way communications like chats and emails, is even fewer: 4 for men and 6 for women.