To get a sense of how that would work, check out #InteroperableFacebook, a video and essay I put together with @eff to act as a kind of #DesignFiction, in the form of a user manual for a federated, interoperable Facebook:

https://www.eff.org/interoperablefacebook

He points out that this kind of mandatory interop is a preferable alternative to the unconstitutional (and unworkable!) speech bans proposed by #Florida and #texas
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Interoperable Facebook

In this video, we discuss the forces that keep us using services like Facebook long after we stop enjoying them (hint, it's not because social media is "addictive") and we present a short "design fiction" explaining what it might be like to use social media in the near future, after big companies...

Electronic Frontier Foundation