Wikipedia offers this picture of ActivityPub:
Ward via matrix 
We differ in that we reject the notion of an inbox. Rather we see the federation as a collection of outboxes from which actors are free to draw from at their convenience. The distinction is one of me pulling from your outbox rather than you pushing into my inbox. We might ask, in our system how can your algorithm decide what I am to see? Short answer, there is no place in our architecture for your algorithm. (Here I use algorithm in the modern negative sense.)
mike_hales Each would need characterising - and I don't know whether 'protocols' is the right way to approach this. Might be. Certainly is the way that federators like the ActivityPub folks do it. The protocols of the humans differ from those of the machine. Significantly? Perhaps the protocols of humans-as-writers are distinct from those of human-with-human federating?
paul90 The difference is a lot deeper than our rejection of the notion of an inbox. Maybe worth adding in the words from the W3C standard > The ActivityPub protocol is a decentralized social networking protocol based upon the [ActivityStreams] 2.0 data format. It provides a client to server API for creating, updating and deleting content, as well as a federated server to server API for delivering notifications and content. In Federated Wiki, there is no server to server linking. To all practical intents, for the reader, the server content is static/inert. The connection (federation) between wiki is based on readers journey through the content, is context based and ephemeral.
See Secure Scuttlebutt. (gossiping)