@evan While writing about European privacy laws, I realised that any Fediverse research that uses personal data (even the public data) would have to be on a strict opt-in basis for European users.
When you subscribe to a social network (Fediverse-based or otherwise), you always have to agree to its terms of use, which also have to tell you something about how your data will be used. If there is no mention of (academic) research, then no one is allowed to use my data for that. Data like "who interacts with whom" (the social graph) is declared as very sensitive data, so it can never be used for anything other than the intended purpose (communication) unless I explicitly agree.
The fines are really high. Meta, for example, has had to pay fines of more than 2 billion euros in recent years, Amazon almost 800 million euros, and so on.