I just stumbeld across a nice paper that combines cell-phone records and surveys to understand how friendships change over time and how many close friends we have. The results "[...] are likely to reflect limitations in the ability of humans to maintain many emotionally close relationships, both because of limited time and because the emotional “capital” that individuals can allocate between family members and friends is finite."
So, Facebook is one thing, real life is another. Ok, this may not be spectacular news for many, but scientific research into the differences between online and offline social network formation is rare, and this is why the paper appears to be interesting.
Persistence of social signatures in human communication, in the Proceedings of the National Academy of the Sciences (PNAS)
A short introductory discussion on Sciencemag.