I'm on Facebook, too. I don't think anyone can alter my profile but me-- but hey, what do I know? I created the account to have a professional presence easily accessible on the web, in case any agents or editors want to look me up. (I also have a web page, but it's out of date.)
MySpace seems to be for the younger/music/artist crowd, Facebook seems to be more friends/family/professional. I have most of my contacts set up there, and on LinkedIn. I started on MySpace, but Facebook gets more traffic. I really don't use it as a communication tool, though-- I chiefly use email for that.