Thanks for the clarification.

Well, I'd expect Meta to drag their feet as much as they can, tbh. So: Years and as many "regrettable" technical hiccups and UX inconveniences as they can get away with without having to pay too stiff a fine. Same as always.

I am aware of adverserial interoperability, but, frankly, it's one of those ideas that make me chuckle benevolently. I don't see much practical merit in it. As for Facebook getting big that way in the first place: I strongly disagree. They got big by being early, good enough to capture the zeitgeist, and then being as anticompetitve as they could. Just like Microsoft before them, for example.