I'm sort of tired of talking about this, particularly because it's all academic: they don't offer any federated instance currently to defederate from. But I want to clarify my reasoning.
My point is NOT that we should "gIvE tHeM a ChANce!" We all know they are run by greedy, exploitative, untrustworthy people.
My point is that I think our response should be rooted in causes and effects rather than vibes. If they offered a federated instance that I had friends on, I'd want to be able to see my friends posts and have them see mine. And if Facebook then tried to find ways to push ads to a remote instance, then we'd obviously defederate. Because that's an appropriate tool to correct a specific harm.
That's all I'm saying. Currently, Facebook has no means of affecting my experience on this instance, so I don't care whether we preemptively reject them from federating. The moment that they DO affect my experience, they'll obviously be booted. That's good enough for me.