My parents and lots of aunts/uncles/cousins use Facebook, so for me it's always been a good way to keep up with them, keep them in the loop, post some pictures of the kids, and all without having to make dozens of phone calls or whatever.
Although I'll admit as the kids have gotten older, we're having fewer photo ops, so my updates have grown less frequent. Might be approaching a time when I'll abandon it as well.