I think a difference between email and ActivityPub-based social media is there's arguably less of a need to have federation between any two servers. If you can't email the government, your sister living abroad, or a client, that's a big problem. But if you can't follow a cat pictures account or your friend's constant stream of baseball rants because the servers don't federate it's not quite the same.
If Meta becomes ActivityPub interoperable instances may or may not federate with them. Either way it's not necessarily going to change my social media experience.