Ya most of my friends have both.....I just like to be able to change my profile and mess around with it and such. Prob because I had a myspace before I had a facebook. They are very different, and most people like one better than the other. To each their own!
Facebook is a social utility (rofl) where you add people to your friends list and customize your profile to reflect yourself. Its for just about anyone really. I know middle schoolers and adults who use it.