I may I have said this before, but the most honest reason I think I don't have one is that I have some sort of thing where if enough people are doing something (i.e. Energy drinks and cell phones), it kind of backfires in popularity in my thoughts and I don't want to do it because it gives me this uncomfortable sense of conformity.
That, and I don't exactly feel comfortable exposing a bunch of information about myself to the entire world (and the Fox Corporation). Yes, I'm aware you can make it "friends-only", or whatever, but my friends already know all the stuff about me that I'd ever post on a MySpace (also, kudos to "Tom", or whoever--I heard he lied about his age for one reason or another--for giving a website a name that no one has any idea how to pronounce in a possessive context).
Besides, if I want internet socialization, I've got it here. Yeah, not a whole lot of people like me, but I'm used to that in my life in general by this point.