Dunbar’s number is the nominal limit to the number of people with whom we may comfortably maintain stable relationships, aka friendships. The number is proposed to be between 100 and 250, with a commonly used value of 150.

This number was first put forth in the 1990’s by British anthropologist Robin Dunbar after studying the relationship of primate brain size to average social group size. In informal terms, he describes it as, “the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.”

More from Scientific American…
Humans are extremely social creatures. Anthropologists maintain that our hypersocial nature has helped us become a uniquely dominant species. Now social media allows a large percentage of people to communicate effortlessly worldwide (see large graph), something no other animal can do.

Yet despite running up hundreds of friends on Facebook and thousands of followers on Twitter, we are fooling ourselves, scientists say. We can really only maintain about 150 meaningful relationships at any time. Study after study confirms that most people have about five intimate friends, 15 close friends, 50 general friends and 150 acquaintances. Robin Dunbar, an evolutionary psychologist now at the University of Oxford, who had showed this pattern convincingly in the 1990s, revisited his old conclusions in a recent study of several thousand Facebook users. He found that despite social media’s explosion, our network of significant contacts still maxes out at around 150. This threshold is imposed by brain size and chemistry, as well as the time it takes to maintain meaningful relationships, Dunbar says. “The time you spend,” he adds, “is crucial.”

• Only 150 of Your Facebook Contacts Are Real Friends – Scientific American
• Dunbar’s number – Wikipedia