You thought I was going to say MySpace, didn't you? I'm going for quality here, not quantity. Sure MySpace has a lot more people, but they don't have the things I'm looking for in a social networking site. Fun!
Facebook is full of fun applications that you can add to your Facebook profile. Play games with your friends, battle each other, send gifts, create virtual pets or virtual gardens. There are just so many things you can do and so many options you can add that you'll never run out of fun things to do.
Although you can't modify your Facebook profile to look any different than it does, you can create a photo album, blog and all the other things a great social networking site should have.