The main problem I have is things that are done outside of school or work seem to carry a lot of weight. For example, even if you are 21 and just have a glass of wine with dinner, if a picture goes up on Facebook and a future employer sees it, you may be judged. I'm not sure if I believe that's fair. I am studying to be a teacher. I would never drink alcohol around students or do anything inappropriate, but if I'm having a glass of wine with my friends at dinner and a parent sees a picture posted, I may lose my job.
I think that Facebook is becoming more and more of an issue, and for the sake of argument, I think it is better to safe then sorry...when in doubt, don't post it.