What is really required is a database protocol for tracking Likes, or "a client/server API for creating, updating, and deleting content, as well as a federated server-to-server API for delivering notifications and content."

But they have that! I didn't know about ActivityPub until I read mxuribe's comment above. That's a good start.

As mxuribe says, "having an existing standard doesn't mean that the Facebooks's of the world will choose to adopt it." I would expect FB to resist it. But if the backlash and dissatisfaction with FB grows, a protocol like ActivityPub is a necessary enabler for something new to happen. By allowing multiple providers to share content in a federated model, the protocol could grow organically without requiring one big new player to migrate all the FB users to a new monopoly.

Once it starts to happen, FB customers could be bridged into the new federated universe with translators that mirror content from FB into the new ecosystem.