Using Firefox and Adblock Plus, you can block some unwanted “features” of Facebook: http://blog.makezine.com/archive/2010/05/how-to_avoid_facebook_malware.html. For example, it should prevent Facebook from sharing information with other websites.
Or if you want, you can completely delete your account, thanks to the The Consumerist. You could also just deactivate your account if you plan on re-using it.