> On one hand, forcing Facebook to open up its social graph would have serious privacy implications.

There shouldn't be much additional privacy concern with a Facebook user saying "I trust user Alice and service example.com, so please send all my posts to alice@example.com using ActivityPub."

Similarly, Alice should be able to tell example.com that she trusts the Facebook user Bob with ID 123456789, so that example.com sends her posts to Facebook addressed to Bob (perhaps authenticated with a pre-shared key that Bob emailed her).

Once users are free to move between providers, those providers can compete based on their level of privacy and security, which right now Facebook has little incentive to improve.