Why? It's an awesome way to keep up with old friends that have spread out in the world; friends that you never wanted to give up, but they moved on or you moved on. It's a great way to keep up with current friends that you don't have enough time to get together with on a regular basis. it's a good way to communicate with friends you do have time to get together with regularly. It's a good way to stay informed on what friends and family are doing. It's educational if you have friends that have interesting lives and post interesting stuff. There's alot more plusses to it, but it does depend on how many people you are friends with, and how interesting their lives are.
If a person only has a trickle of friends in real life, Facebook isn't going to work because it does need to be brought up to critical mass, so to speak.
Sorry to side track the original intent of this thread, but I've read quite a few people around here say negative things about FB, people who obviously have never even tried it. Facebook has actually brought an added layer of communication and integration to a good many people's lives. Of course, I've seen the stories about how Facebook is ruining some peoples lives, but that's only because those people post comments and pictures of themselves doing stupid things. Like anything else in life, there is no substitute for discretion.