The thing about posting on Facebook, or on here, or anywhere really, is not to put something in writing that you wouldn't say if you were standing at the busiest intersection with a multitude of people listening to you. I don't post anything I wouldn't say to their faces so I don't worry much about it. I resisted opening an account on Facebook for a long time and probably never would have opened the account but my stepdaughter asked me to.
It looks like privacy is a dead issue anymore. Certainly, the internet is a place where no one has any expectation of privacy.
That is why I say that I tell people never post anything online that you are not willing to say openly and publicly. Because as you say there is no privacy and people should not expect that their conversations will stay private. In a perfect world, or even in one that was just a little better, we should have the expectation that things we say in confidence in an email or in a private message would indeed be private but I never assume they will be. I always assume that whatever I say can and may be made known to those I speak out against. But I am just as willing to say what I say about them TO them.