Do online social network reduce real-life communication?

Last weekend I read an interesting piece in the New Yorker about a new course at NYU that teaches student how to network in the real world - i.e. meet fellow students. It states the example of one freshman who already counts 900 friends on Facebook - when he starts meeting some of his "friends" he doesn't even recognize them.
The article made me think about my real-life social behavior in the age of online social networks. I'm an active user of Facebook, Linkedin, Xing, Gtalk, Asmallworld and occasionally Twitter. Whereas Facebook and Asmallworld are mostly real friends (being defined as people I actually know in real life), Linkedin, Xing and Gtalk are mostly business contacts and Twitter is purely experimental.
My time on the phone with people has definitely decreased over the last few years though - mostly towards email and SMS though at this point.
It would be interesting to get some research data on this question or simply other opinions