But there is also data that I may not want to be public knowledge. If Facebook scans my chat logs, then Facebook knows a lot about me that I would not want some of my friends and family knowing. And I wouldn't be surprised if their TOS permits them to scan my chat logs.
And you can also do interesting oracle attacks with this. Let's say I'm trying to figure out if my son is homosexual. (Substitute for any fact Facebook may know, and for any person you want to investigate). I already know a ton of things about my son. Age, where he lives, what sports he plays, his favorite foods, etc. So I create one really specific ad that's guaranteed to include only my son and maybe a few other people. This one is only shown to homosexuals. Then I create an alternate ad that targets the same group, but only heterosexuals. Then I just need to watch which ad appears on my sons computer, and I suddenly know what Facebook thinks his sexual orientation is.