Thank you for starting this!
I must say the original proposal involving Facebook analogy gets past my understanding: I have no clue what Facebook does, and I do not understand how it relates to Discourse in any way.
My understanding of ActivityPub support for Discourse would be that it can help federate topics, or even share a category among Discourse instances. For example, one announcement topic on discourse.joinmastodon.org could be federated to socialhub.activitypub.rocks in the related
#software:mastodon category: there, local users could like, reply, quote, etc., as if it were a local topic, except the original topic would live on joinmastodon’s instance.
Another aspect of it is that if one has an account on both instances, there should be a way to link these accounts together, i.e., to use one specific Discourse instance as the main identity provider. I understand that this is not the focus of a first iteration, but it’s good to keep in mind, since we could end up having “sign in with [put your favorite ActivityPub implementation here]”.
What I understand from the proposals above is a replication of the Discourse app on Android, where you have a list of instances, and get notifications from all of them. It seems a bit dangerous to interleave unrelated responses from many sources, especially as they lose context.
Did I understand correctly, and would my understanding make a good second step for your vision of integrating ActivityPub with Discourse?