The problem is that these platforms have, in a very meaningful sense, taken the place of communications infrastructure. The bulk of us use Messenger or iMessage

The problem is that these platforms have, in a very meaningful sense, taken the place of communications infrastructure. The bulk of us use Messenger or iMessage/FaceTime or some other over-the-top service for things we would have otherwise texted, or sent over email, or called someone about using a dialer, and that contributes to the vendor lock-in that they have. It's the new IRC, or AIM, but with an even wider capability gap and a userbase billions deep. Even RCS doesn't solve this, because they're still a technically inferior spec - they don't support E2EE, and they still depend on phone numbers, with the terrible user experiences of arbitrary last-century identifiers and the phone companies you (generally) have to subscribe to to get them. Until social media companies or their chat protocols are forced into some sort of interoperability - a thing we can do, under antitrust law - no amount of data portability will fix this. Because it's a consequence of their owning our networks, not just our content.