It started with old friends from high school, which was sort of nice, but obviously if I'd have really wanted to keep up with these people, I would have made more of an effort to do so.
But now it's the only way that the majority of my friends communicate. None of them use email anymore, almost none of them have blogs, or websites.
I can totally understand why nerds find Facebook's popularity confounding (as they were already doing all of the things that it facilitates in a more customizable fashion without the lockin of one company), but for every single normal person I know, it's the de-facto way they use the internet.