I have a facebook which I use for myself, my forum has a facebook page and a twitter account which are used to post updates to the forum and for promotion.
I'm on facebook, mainly to keep up with family, but I have tons of friends on there too. I have myspace too but it is dead because facebook pwns myspace... I ever so rarely update twitter from my phone.
I have a Facebook account, but I hardly use it. I have a Facebook page for my forums though, which has proved fairly popular and seems to work well to drive traffic to the forum, esp when there is important announcements, same with Twitter