Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
I have no idea what’s the current state of the art in such profiling, but I’d assume that what content you consume should be enough to build a pretty good profile about you. And if they manage to link your profile name with the rest of your data on the internet, which unless you’re using a VPN and robust anti-fingerprinting solution wouldn’t be too hard, then it may be valuable to someone for something, who knows.
Or maybe not, the whole “what are the corps doing with the data” is still a mystery to me. On the other hand, I’m sure you could build a ML model that would take the upvotes and downvotes as an input, and figure out what best to advertise to you on other platforms. Or what to message to you to eventually manipulate you into some end game goal.