I think moderation and community are hard to tack on, though those might be necessary to make these projects as popular and usable as social networks, to get the network effects rolling. Without them this risks becoming either "4chan everywhere," or "your one comment no one ever reads everywhere."
Genius had an annotate everywhere feature that ran into the moderation issue: observer.com/2016/03/genius-web-annotator-emma-dawson-alana-massey
Probably out of scope for you, just think those are interesting untackled problems in this space.