If they are storing a this data anyway to make the thing work (so you can go and un-upvote the thing you upvoted five years ago), how is “privacy” reduced if they also have the as system decide to show you an ad because people who upvoted that thing tend to click on the ad?

No new people or businesses are being given any new information bout anyone. Is it because what used to be a passive database is now starting to think, and your privacy is infringed because the system itself is now looking at you when you didn’t expect it to?