Saw an interesting post on Planet Ubuntu about whether Debian needs to be on Facebook.
Obviously a lot of people who like Linux are on Facebook, but when a large portion of the developed world is on there that's inevitable. Not that it means they are getting useful information about Linux. Whoever owns the Debian page is doing nothing with it. I've found it common that certain pages can have thousands or more people 'liking' them, but there's no involvement.
That said, there are many people for whom Facebook is the main thing they use on-line. They can see what their friends are up to and get updates from bands or other things they 'like' with none of that messy email or RSS stuff.
I'm on Facebook because enough people I know are on there to make it useful. I can reach them via it, e.g. with interesting links or automatically when I upload music I've made. They can respond there if they want without having to register on yet another site.
I don't think Facebook is perfect by a long shot. Lots of things about it annoy me and I think people who mostly use it are missing out on a lot, but for now I'll exploit it for all it can give me.
There are people working on more open alternatives suck as Diaspora, but a social site is of limited use without a critical mass of users and it's going to take a lot to get people to move away from Facebook. identi.ca is big enough to be useful, but I still use Twitter to reach others.