[vidarh]

To add regarding the signatures and portability, Mastodon at least signs its posts with JSON-LD signatures. They're in the downloaded archive I got from mastodon.social when I moved off it, for example. So is the key required to validate them.

The only thing missing there to make this better - and this is not an ActivityPub thing - is including those JSON-LD signatures in more contexts, so that you don't need to rely on the export functionality (of Mastodon) to get an archive, allowing clients to choose to keep a local copy (or nominate someone to back it up for them), and providing an upload functionality for posts (Mastodon doesn't do this, but that's also a Mastodon thing, not an ActivityPub thing).

I wouldn't have had an issue if they added extensions to ActivityPub. There are even things Mastodon refuses to add that I'd applaud people for forcing the issue on by adding extensions to support. But their choice to reinvent everything puts me off.