I attended DefCon 16 and one of the talks was titled, "Satan is on my friends list". Catchy title, huh! Anyway, I read a blog posting from Rich Mogull talking specifically about this talk. I didn't attend this particular session, but I've heard about this from another Security Podcast I listen to called Pauldotcom. There are known instances where someone signs up on some of these social networking sites and steals your identity.
If you think about it, if you are known in your community and you have built up a reputation, you don't want anyone to imitate your identity. A recommendation is to go out to sites like MySpace, FaceBook, LinkedIn, Twitter, and others I may not have mentioned, and sign up. You don't need to actually update the sites. Just get your name listed so nobody else can steal your online identity.
There are those with not so great intentions out there who will do this and take advantage of your reputation to possibly gain some information from unknowing people who think it's really you. So if you haven't already, go out and sign up. Protect yourself today.