I haven't thought much about the lower level network stacks but I think social media platforms of a certain size ought to have certain legal requirements to do things that incentivize interoperability or allow competing platforms to emerge (perhaps non-commercial ones.) Primarily things around APIs and data formats.

So a regulatory body that would enforce secure, private, and open sharing of user data between services at the user's behest? Sounds good to me.

Yep pretty much. If you support this, you should know that if this ever gets legs despite its surface level appeal you will see a lot of people not supporting it, because it's exactly the kind of thing that led to Cambridge Analytica. So that'll be a good time to truly ask yourself which side of this tradeoff is the one you want to make.