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.
But it’s the fact that it’s so easy to reverse engineer. You can take that data to build a large profile on each user in your instance (or possibly all instances)
Sure, this can be abused. Build profiles on each user based on what they are talking about and what communities they subscribe to. Then start sending them direct Lemmy messages with ads or links to shitty or dangerous sites.
The fact that anyone can start an instance and get all this info sent to them is a bit unsettling. But we will have to find a way to protect against it.