Do you know how to use Facebook's privacy settings? Even if you do, did you know that Facebook will turn "turning your info over to third parties, just because one of your contacts takes the “Which Gilligan Island character are you?” quiz?"
Wired writer Jim Merithew has written an article about Facebook's less-than-innocent privacy practices. I suggest you read it if you use Facebook, even if you think you're safe. Actually...especially if you think you're safe.
The sad part is, I don't see a large number of people deleting their accounts from this. Hell, even I can't. For a lot of people, Facebook is a critical site for connecting with friends and companies.