Facebook cannot, because Nostr has a low-key bitcoin anarchist vibe that Meta will be super wary of. Whether it’s all on Meta servers or not is irrelevant to the headlines that their PR team would have to deal with. social protocol integration decisions are rarely ever made on tech alone. It’s almost always conflated.
Another way to illustrate the conflation is the sentence “ I can’t wait to share this with Nostr!” This means something. We can all picture who might be saying this and what sort of thing about. Nobody says “ l can’t wait to share this with html”.
ActivityPub is probably about as far as Meta can go.