The size of social networks | Primates on Facebook | The Economist
http://www.economist.com/science/displaystory.cfm?story_id=13176775

"What also struck Dr Marlow, however, was that the number of people on an individual’s friend list with whom he (or she) frequently interacts is remarkably small and stable. The more “active” or intimate the interaction, the smaller and more stable the group."

Here's The Economist article on FB. The best quote: ...people 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,” says Lee Rainie, the director of the Pew Internet & American Life Project.