My Facebook is "private" in the sense that I only allow friends to see my posts.

Plus anyone with access to your friends' accounts, plus whichever of THEIR friends, spouses, coworkers, etc, happen to be looking over their shoulder as they view your so-called private posts. And any image can be saved, any post can be copied and pasted.

Remember, everything you do on the internet is there forever, and eventually everyone will see it.