Primates tend to maintain social contact with a limited number of individuals within their group. But here's the thing: primates with bigger brains tended to have a bigger circle of friends. [Anthropologist Robin] Dunbar reasoned that this was because the number of individuals a primate could track was limited by brain volume.
Then he did something interesting. He plotted brain size against number of contacts and extrapolated to see how many friends a human ought to be able to handle. The number turned out to be about 150.
Since then, various studies have actually measured the number of people an individual can maintain regular contact with. These all show that Dunbar was just about spot on (although there is a fair spread in the results).
What's more, this number appears to have been constant throughout human history--from the size of neolithic villages to military units to 20th century contact books. ...
[Bruno Goncalves and coauthors] studied the network of links created by 3 million Twitter users over 4 years. ...
It turns out that when people start tweeting, their number of friends increases until they become overwhelmed. Beyond that saturation point, the conversations with less important contacts start to become less frequent and the tweeters begin to concentrate on the people they have the strongest links with.
So what is the saturation point? Or, in other words, how many people can tweeters maintain contact with before they get overwhelmed? The answer is between 100 and 200, just as Dunbar predicts.
--Physics arXiv Blog on friendship capacity. HT: ACT