The difference between E-mail and Facebook is that email isn't designed to be social (aka it isn't designed to spread information to masses of people quickly), while Facebook is.
However, I still don't upload pictures I'm not going to be proud of, even if it's just for an email to someone. Once it's sent to someone else, I assume that it could leak to the web in any number of ways.
Perhaps that line was too harsh, but the point is that Facebook doesn't control what your friends do with the information you post on your profile. Thus, Facebook really doesn't have the ability to keep your private stuff "private," and it's dangerous to assume that they can.