I also wonder how big the overlap is between people who would use a federated platform and those who would willingly use anything made by Facebook.
It doesn’t have to overlap if they bake it into their existing website. A huge portion of humanity has a Facebook account, even if they don’t use it. They’re baking in as much as they can with Marketplace taking over Craigslist’s former space, trying to capture VR with your Facebook account, and now they want to take over Twitter’s space. And I’m not saying the backend work wouldn’t be huge, but their whole “posting stuff to people who follow you” schtick fits perfectly with the Fediverse. There’s nothing stopping them from just federating everything.