Why is everybody joining Facebook?

I've always had the impression that Facebook was basically a forum where you created a public advertisement for yourself and tried to get as many people as possible to "friend" you. As a sideline activity, you can apparently leave inane comments on your friend's sites and publish embarrassing pictures of the small subset of "friends" that you actually have met in person.

This sounds pretty awful to me, and I've been pretty much ignoring the whole phenomenon. But more and more people that I know seem to be joining. Maybe I've had the wrong idea about Facebook - maybe mixing it up with Myspace? I'm willing to revise my ill-informed first impressions. Have you joined Facebook, and what do you like about it? Should I join?