So why the fuck would anyone want to be on facebook?
People who can think more than 5 minutes ahead don't want to be on facebook, and they aren't. However the majority of people only care of "now", even if that much. They post a message or a photo because they can't be bothered to think what it can cause in the future.
To make things worse, it generally hurts your privacy to have a social life these days. Even if *you* didn't take the photo and didn't post it for all to see, someone else - out of tens of random people you met - can do so and attach your name to the photo, all that without you knowing. And that's how the data trail grows.
Even on /., formerly a bastion of near-paranoid sysadmins, posts appeared (a few years ago, IIRC) saying "the war for privacy is lost, so abandon all hope, drop all defenses and have fun!" And there is indeed a serious push to abandon all hope because it takes more and more effort with every passing day to keep your privacy, and it certainly puts constraints on how you deal with your friends and how you participate in social events. I do not expect majority of the population, especially teenagers, to sacrifice that much at cost of what they value most [at the moment] - their friends and their social life.