If you don't know what it is, basically you can look up people from your past, that you've lost touch with and reconnect (if they use facebook too, but right now it seems almost everyone does). There are other services like this (classmates, gradfinder), but facebook it miles better then those two combined. And it's free. My wife started using it about a week ago and commented how addicting it is. I used her account to look around for a day or two before getting my own account.
I refuse to use the word 'addicted', but I certainly am committed. I cannot believe how easy it to find people. People from every part of your own personal history. Grade school, high school, college, family, extended family, lost friends, long lost friends, co-workers, former co-workers, even ex's. Not that I have
For example: The day I signed up, I found my childhood best friend. A guy who I spent everyday with from kindergarten until the end of high school when we went to different colleges. I haven't seen or heard from him in at least 13 years and we've chatted for the last couple of days. If you played a lot of Tim Horton's Roll-up-the-rim and doubt that anyone ever wins big, well some people do. He won a Toyota Hybrid, but he doesn't drive so he sold it!
I give two thumbs up to facebook.