I ran across this post about how someone could potentially steal your identity using a social networking site like Facebook. He actually uses Facebook as the main example. It's interesting to see how much information one gives out on Facebook without really realizing it. But, I think he goes a bit overboard.
Is it really possible for someone for steal your identity from the trivial information you scatter around Facebook? I suppose so, but it would be tons or work. Hacking and prying and keeping track of everything. Would take months, or longer.
Would it be worth it? I guess it depends on who's identity they were stealing.