Family and friends. It's where people congregate to share pictures and talk about life and I like doing those things.
I'll admit that I'm not a big FB user, but I still think there are plenty of good reasons to use it. We're social creatures and a lot of the people we like to be social with are on there and not available for social interactions any other way. I have friends and family members who I would never communicate with if it weren't for FB. That's not a bad thing.