I don't get why this is creepy. They already have your data (you're posting it on their site after all!) Its no secret that analyzing data is something that these "information" companies can and pretty much always do.
I could see being creeped out in 2010 when this kind of shit was new but by this point it should be considered the default position. You should almost be more creeped out if they're NOT doing this kind of shit since it kind of suggests they're probably doing something even more nefarious (not that they couldn't do both of course..)
Now pissed off that they're going to start injecting ads into your conversations? That's something I can get behind. Or at least I could if I actually used FB.
It's creepy because people have an expectation of privacy. Like when you call someone on the phone, you expect it to be between you and that person. You know the computer takes care of everything, so no reason why anyone should be listening or recording it. These same people think they should be getting the same sort of privacy with messenger apps.
Of course, any one paying attention to the last decade knows that all our shit is being recorded, for some reason or the other. No matter what you wanted, we are in a surveillance society, the question is, do we make laws to protect us from ourselves, or do we just carry on how we are doing it?