@john the thing about this that bugs me is that Flickr already supported independent federation via Atom and RSS feeds, and IMO ActivityPub is not a very good Atom/RSS replacement. Mastodon used to be based on Atom+WebSub, but then switched to ActivityPub not due to any real technical improvement but because it was easier to hack in private posting (badly), rather than working with any of the initiatives that existed to add post privacy to Atom.