I read this a few days ago:
http://www.wired.com/vanish/2009/11/ff_vanish2/
Privacy is even more dead than I ever suspected.
As many have pointed out, this happens already in blogs and email, on twitter, in various forums, etc., and not joining facebook will do nothing to prevent it from happening on facebook.
But more importantly, if you don't want your friends doing X or Y to you on facebook, you have a very powerful method of ensuring that at your disposal: make it clear that you find this unacceptable when it happens. Over time, people get the message before making the mistake, and these expectations harden into social norms. Ultimately, relying on these social norms is much more robust than relying on technological solutions, in my opinion.
In fact, I would go as far as to say never put any critically sensitive information on the web.