"I think that most instances would block Facebook if they tried to do something like that."
I don't think so. Doing so would mean that their users wouldn't be able to get events from the biggest network; the one where most people are on. While today, most users of ActivityPub are likely to be people who are against Facebook and not wanting to interact with it, if ActivityPub becomes huge, then more people would be using it that are ambivalent about FB or even users of it. Blocking FB would mean that those people leave the instances, because they're no longer able to get a significant amount of their messages.
I've thought about it more, and it's a tough question. If Facebook did allow Facebook users to see and interact with ActivityPub posts, but didn't allow other ActivityPub instances to see Facebook posts, there would be three types of instances.
There would be FB itself, of course. Then there'd be AP instances, short for ActivityPub instances that don't block Facebook. And finally, there'd be AP-FB (AP minus FB) instances, short for ActivityPub instances that do block Facebook.
FB accounts can interact with AP but not AP-FB. AP-FB can interact with AP but not FB. AP can interact with AP-FB, and only partially with FB.
If there aren't many people on AP-FB instances, then the incentive for people on AP is to jump straight to FB, because that's where you have the most reach.
If there are a lot of people on AP-FB instances, then there's some incentive to jump to AP accounts, because then you can interact with AP-FB and Facebook people can see your posts, even though you can't see theirs. It's not clear how strong this incentive would be, though, because people being able to see your posts might not matter if you can't see theirs.
If every instance is AP, they lose users to FB, so they have an incentive to coordinate to make a strong AP-FB block. But if there's a strong AP-FB block, AP-FB people would have incentive to jump to AP. This is likely to stabilize with FB winning, if the incentive is strong enough, so we could have a Bad End.
This means the correct game-theoretic solution, as far as I can tell from my armchair, would be for the AP-FB block to coordinate and block not only FB, but also AP instances, so that there'd be no incentive to jump to AP.
So it would all depend on how well instance owners can coordinate, and how willing they'd be to block AP instances, who could be framed as "defecting". I have no idea how that would turn out; humans have been known to coordinate, but also been known to fail to coordinate. It's something we'll see in the future, if Facebook ever partially implements ActivityPub.
Well, what would probably happen is that Facebook interactivity would be some kind of plugin, which instance runners would have to enable (and possibly configure). Convincing them to do so may not be easy.
I don't think so. Doing so would mean that their users wouldn't be able to get events from the biggest network; the one where most people are on. While today, most users of ActivityPub are likely to be people who are against Facebook and not wanting to interact with it, if ActivityPub becomes huge, then more people would be using it that are ambivalent about FB or even users of it. Blocking FB would mean that those people leave the instances, because they're no longer able to get a significant amount of their messages.