Facebook was a wonderful way for me to keep in touch with a lot of different people, but I deleted my account a few weeks ago.
By the time I deleted my account, Facebook had given me A LOT of reasons to leave: their increasingly complex and deliberately obtuse privacy policy and options, the frequent changes that defaulted to making information I was only interested in sharing with friends public (like status updates), making private information public without my knowledge, making it impossible for me to keep some information private without deleting it, making it difficult or impossible to remove some information, and using their terms of service to stifle innovation and (terrifyingly) bring criminal charges.
I know that my leaving Facebook has absolutely no effect on the site and isn’t going to influence their policies. That’s not why I left: I left Facebook because it offended my ethics and endangered me professionally.
If you’re worried about privacy on facebook, but don’t want to delete your account, try this tool to help you fix some of the privacy issues. Otherwise, delete your account and check out the Diaspora project.