Something like court-mandated federation is not completely out of the question in the EU, given that the GDPR requires "data portability" between digital services, including "the right to have the personal data transmitted directly from one controller to another, where technically feasible."[0]

I suspect a big American corporation like Facebook would have a hard time persuading an EU court that it wasn't technically feasible for it to automatically duplicate your Facebook posts onto a competing Fediverse instance where you have an account, and the court could even decide that the "natural" technological implementation would be to broadcast your Facebook posts directly to your friends across the Fediverse.

Sadly the language of the GDPR seems to only mandate the export of personal data from the site where it is stored, and not grant the complementary right to have data imported. This means Facebook wouldn't have to show you the posts of any of your Fediverse friends, and it also wouldn't export your Facebook friends' posts to be viewable on your Fediverse account (unless they also had a Fediverse account and chose to export those posts themselves).

[0] https://gdpr-info.eu/art-20-gdpr/

I'll settle on just enough to have a viable healthy, open and free tech ecosystem. My evil regime will even let you keep the rest - as long as you play along nicely and interoperate and federate!

Anything built by appropriating other people’s property and/or by depredation of the commons.