Many of us have grave reservations about loss of privacy by use of Facebook accounts. Others have serious complaints about Facebook's ever changing privacy policy, and their evolution of new features where you're opted in unless you opt out, and often by the time you have a chance to opt out your privacy is already breached. And you never know when FB will change their policy again, or when new features will give you new privacy leaks.
Most of us who are not on Facebook (me included) just don't trust them to maintain our privacy, and in fact many of us see their site is the opposite of privacy. A feature for many, a problem for a few who still value our privacy.
I don't see how you can join FB without ruining your privacy.