I've joined Merlin and Mathowie in deactivating my Facebook account. I mostly did it because Facebook wasn't useful to me but the incredibly hostile new beacon feature and their new social advertising efforts were the last straws.
What asshole bizdev person at Facebook thought it'd be a good idea to let third party sites implant your purchase history on Facebook? Tracking user behaviour across third party sites is wrong. Ignoring an opt-out popup for 20 seconds is not evidence of informed consent. We used to brainstorm ideas like that at Google but the basic "Don't be evil" ethos always squished the discussions I was in. Facebook apparently lacks a moral compass.
After the initial novelty Facebook stopped being useful and just became annoying. A particular problem was the zombie superpoke spam, new crappy viral applications that popped up every week demanding my attention. I care very much about my social relationships, but I don't care to maintain them through a website that reduces my social contacts to random bits twiddled in childish toy applications.
In true Web 2.0 form there's no way to actually delete my Facebook account. All I can do is mark my account "deactivated". They even helpfully told me I'll still get invitations to join crap. Facebook isn't just a crazy ex-girlfriend, it's a crazy ex-girlfriend who follows you around when you're shopping and whom you can't get to stop calling you.