Another thing people find surprising:
Self-hosted Reddit servers used to be commonplace.
Now imagine if ActivityPub existed when Reddit was open source.
Well, we don't have to imagine it. There's Lemmy.
But my point is that every self-hosted Reddit install could have been federated if ActivityPub existed back then.
I completely forgot that #Reddit was #FOSS! And in fact I think that old code, or a fork of it, is still used to host forums to this day.
Imagine if somebody forked it to support #ActivityPub…
Granted IMHO all #Fediverse forum platforms are going about this all wrong; instead of modelling their platforms after #Reddit, servers should be structured as communities in their own right. i.e. instead of a #Lemmy instance with communities for #Politics, #Gaming, Tech, #Sports etc., there should be a #politics instance, a #sports instance, etc. That might be hard to implement though.