You're being overly complacent. Facebook might find it harder to track people who don't sign up to it, but they still have shadow profiles for tracking non-users. You get tracked by what your friends and contacts share about you as well as what you choose to share. If anyone who put you in their mobile contacts let Facebook's app loose on their phone, then smile, you're already on Facebook.

Privacy is an environmental issue, not a transactional one. With the current system, there's really no opt-out short of opting out of social life altogether.