According to Oxford University anthropologist Robin Dunbar, who has studied human relationships in societies from the Stone Age to Facebook, most people are capable of maintaining stable relations with about 150 others. Within that 150 – a figure now known as Dunbar's number – Dunbar has identified smaller concentric "circles of intimacy."

Healthy people, he says, maintain a 10- to 15-member "sympathy group" (the death of any of whom would cause distress) and three to five close friends who can be relied upon in times of trouble. If your own friend count falls within that range, you're doing fine.

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Unfortunately, that's not the case for most of us. According to data compiled by the University of Chicago's General Social Survey, between 1985 and 2004, the median size of networks of personal confidants – groups of genuine intimates – decreased from 2.94 people to 2.08. Worse still, the GSS found that in 2004, 25 percent of Americans reported having nobody to talk to at all.