[handrous]

It's close, but browser support & integration was never good enough, and that lacks the ability to carry on a dialog or to attach one's own post to someone else's, which people clearly want.

Any move to develop it further to add those sorts of things would have run into the same problem other protocols have for the last 20+ years, which has made them even more difficult to develop and promote than they already were: keeping huge numbers of users captive has become per se valuable, thanks to having as-complete-as-possible spying-generated profiles on people and massive datasets being important both to the money-spigot of the modern online ad market, and for developing machine-learning solutions to dominate other "verticals" (against any competitor who lacks the same massive, spying-obtained datasets, so is at a huge disadvantage). Application-level open protocols make tracking & spying harder, so companies chasing those datasets really don't want them around. The few that survive in common use do so under constant threat, protected only by luck, by already being "good enough", and by having already established a large presence before that sort of business model really took off.

[EDIT] incidentally, pioneering the inclusion of an open, cross-site social network as a first-class browser feature is one of my crazy ideas for how Firefox could save itself from irrelevance—but like most of the other ideas I have to save them, it's probably too late anyway.