Edit: Been corrected, the following is NOT how it works! Original Text follows
Someone correct me if I’m getting details wrong, but from reading this post it appears as if fediverse admins are provided both the username and email accounts registered by those users that have visited their instances.

If that’s true, one problematic scenario I can imagine is when someone has registered on the fediverse with a pseudonym, but has an e-mail address they also use on their real-life Facebook profile. Visiting a Facebook-run ActivityPub instance while logged in would give Facebook enough data to link both the pseudonymous account (with past and future post history), and the real-life Facebook profile.

So, even if you’re not signed up for Facebook’s version of ActivityPub, engaging with it could still be giving Facebook a source of ongoing data for building personal profiles and targeted advertisement that people would not provide on their own.