This does seem like a genuine problem, and there are already signs of this happening in the Fediverse. Perhaps it needs a name, so that people can notice when

This does seem like a genuine problem, and there are already signs of this happening in the Fediverse. Perhaps it needs a name, so that people can notice when it is happening, and discuss potential solutions to it. The name I propose is "Federator's Dilemma", as it seems like a game theoretic problem. As for a solution, my intuition is that the first step might be to create a Sybil-proof voting system for determining how popular individual services are. If a service is refused federation by other services which collectively contain 90% of user accounts, then that's a sign that the service doesn't do enough to stop its users from posting objectionable content. Similarly, service A could report to service B how many abuse reports it is getting from its users about people on service B, with a clear rule about when service B would be blocked. This isn't a perfect system, and I admit that a Sybil-proof global anonymous distributed voting system might be harder than just solving the Federator's Dilemma more directly, but I think it's worth considering, not least because such a voting system would be useful for countless other applications online.