Half the posts in this thread look like they've turned into some sort of trash facebook campaign...
I think Facebook offers some great features. It's much easier now for when I move out of state to keep up with all my college friends. I'm a guy, and I'm not big into using the phone. So instead of calling every person on the phone, "What are you up to now?", I just check their profile.
The ability to keep contact information up to date is huge too. Just look at that Classmates Finder website. Those people probably wish they had something easy like Facebook; they wouldn't be trying to find out what the hell happened to their old friends.
Professional Appeal -- sort of an unforeseen effect, but if you work in some industry, lets say your a reporter, and you're hoping to get some information about a company, you go to facebook, and oh look, someone you knew works at that company. Instant in. You could also do this if you wanted to work at such a company... etc.