It's the people, not the software...
...that keeps me using facebook. It is a great tool for communicating with friends, when it works, which seems to be less and less often these days.
The clunky interface and horrible layout don't do much for me, but it had achieved critical mass with my group of friends, so I'm stuck with it by default. And their attempts to become more like Twitter are also annoying me. If I wanted to be a twat, I'd use twatter