On top of "message passing" we will build "publish-subscribe" as a second-layer abstraction
"Your ideas are interesting and I'd like to subscribe to your newsletter."
You send me a letter saying you'd like to hear the things I have to say, okay, you're part of the reader list. That's how it works.
On top of that we can build even more abstractions and the net result is that this is how federation works in pretty much every "federated" system I know.
ActivityPub does some extra work to help you see replies on a thread, think "letters to the editor". This is a bit lossy sometimes though
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