Well I think I'm gonna fence sit on this. Personally, I think using facebook a lot is a ridiculous waste of time, but the event organizing and the ability to track (some) people is really useful and well implemented. A friend of mine I had lost contact with some years ago managed to track me down through facebook after specifically signing up to do that.
I only really ever go on if I get an email notification of someone adding me or something... which doesn't happen that often really.
What really bugs me is when people start insisting on using facebook to get in touch - like people who pretty much refuse to use any other method of communication then MSN messenger. I only signed up for facebook to organize a party that I wanted to invite some work friends too, but they pretty much insisted I use facebook to get in touch with them. Oh! and people who you had a falling out with or dont like (anymore) for some reason, who will refuse to talk to you in real life or on the phone or maybe even respond to emails, but will still keep you as a friend on whatever social networking site and even sometimes comment on stuff. But these problems can happen with instant messaging and other types of communication, just seems that social networking sites have the problem really bad.