They would get anything the activity pub protocol gives them, which I can't imagine is much. I'd have to look into it but I assume they're pretty much only sharing the stuff you see.

... Ok so I looked it up (couldn't leave it on the assumption lol). https://activitypub.rocks/implementation-report/ Have a look at the standard here if you like. The only privacy related one I can see is the location header, but it's optional and not implemented by most fediverse apps.

That's not to say it will never become possible for Facebook to get more info. They will absolutely try to throw their weight around if they gain traction in the space, and could, in theory, require more info be shared for fediverse instances that want to federate with Threads. This is why it's important that fediverse admins don't get too chummy with the zucc.

Meta will absolutely try to appear friendly at first, perhaps offering open source development assistance, consulting or training, but it will be a farce to gain power in the space.

Meta are ruthless in the social media space.