From The Economist: It's not 42. It's 148. The magic number for social networks.

Even on the Interweb, we cannot escape our evolutionary past. According to The Economist article: "Primates on Facebook", some of things that we do to/for each other on the social networks over the Internet can still be defined as "Grooming": you need to ping your peeps, follow up on their status, read their Tweets, comment on their Tweets, reply to their Wall because they have left something on yours. These all take time. So does monkeys' grooming each other.

A while back ago, Dr. Robin Dunbar concluded that our brains simply cannot support a social network with unlimited size: think of having to memorize all the names! Probably only Mr. Monk will be able to do that, but of course, he probably has the tiniest social network known to man... Irony, isn't it? Anyway, Dr. Dunbar suggested that the magic number of network limit any animal will be able to maintain is: 148.

Even though in the virtual world, it seems that we can grow our social networks indefinitely, to a certain extent obviously (say, like, 6 billion, the number of entire population...), the average number of "Friends" on Facebook turns out to be 120. And the number of Friends with which Facebookers interact with on a regular basis by leaving comments on their "Wall" is even smaller: 7. That's it. For men. Women are more social, 10.

Even for Facebookers that have more than 500 friends, the number remains relatively low at 17 for men, and 26 for women.

Here is a nice way of explaining this:

"[P]eople who are members of online social networks are not so much 'networking' as they are 'broadcasting their lives to an outer tier of acquaintances who aren't necessarily inside the Dunbar circle'... Humans may be advertising themselves more efficiently. But they still have the same small circles of intimacy as ever."

This also concisely explains what a "social network" such as Twitter represents for a lot of its users, or Tweeple, as they call themselves. Only that it is even beyond the "outer tier of acquaintances", all the way into the nether.