Are you suggesting that someone who wants to publish their latest Facebook post to their Twitter followers needs to export all their Facebook data, select just one specific post from the dumped data, and then import that dumped post into Twitter?

I mean, that's theoretically possible, but it's not the most convenient workflow, and you run the risk that any tooling designed to automate it is frequently broken by deliberately incompatible changes made by Facebook or Twitter (or whichever other services you are using, in the general case).

Fundamentally the data belongs to the users, not to the platforms, so it is right that governments mandate that the data be able to flow according to the users' wishes. All property rights are legal fictions, especially "intellectual property" rights, but at least in this case the property right being defined is a socially beneficial one.

This is not what I mean. I mean make it mandatory for these networks to allow interconnection with eg the fediverse, so that I can connect with people on Facebook or Twitter without having an account on these services myself. Currently users are captive to a network, not due to an impossibility to recover their data, but due to the fact that their friends are using that network and that network is not interoperable. Basically I think networks like Facebook and Twitter are so big at this point that they should be infrastructure.