I have my FB settings up to maximum privacy. However, I have just googled my Facebook name (slightly different from my normal name) and am appalled to see that loads of posts I have made on other FB pages are public.
Also I have been getting spam emails through this Facebook account, I know they must come from FB because I only use this name for FB and they refer to me by that name.
Don't post on any pages or posts that are public (have the globe symbol by them). You can't control how private other people choose to make their pages or posts (shops and companies, for example, usually keep theirs completely public, so that anyone can see them), so anything you write on a page or a post that is shared with the public could be seen by anyone. If you don't want that, then you have to not post on it. You only control the privacy of stuff that you post on your own pages.