But I think "insular" is a funny word to use here, in that it being one giant space meant that self-segregation was a much harder choice to make than on, say, HN or a given subreddit. The thing that always kept me using it was the great variety of voices there. Some of my favorite people to follow were not the ones saying much, but the ones with excellent editorial judgment about who to retweet, exposing me to so many different perspectives.
Early Reddit definitely attracted a pretty interesting, insular bunch but there was never any feeling that Redditors were creating a similar global network. I was on IRC channels that had Americans, Europeans, Indians, Singaporeans, and Hong Kongers but we all knew we were weird. There was always something unique about Twitter users that made them think of their space as a global village, and I think the large presence of journalists and MSM friendliness was a big part of that.
Twitter also managed to create a product that its sticky users loved in a way that I'm not sure any other network managed. Redditors loved Reddit but not nearly as much as Twitter users liked Twitter. Whatever it was I hope product folks study it closely.
Twitter is unique in that it had both wide international reach and no topic structure. Reddit is an engine for shutting you off from 99.9% of the Reddit discussion. Twitter, for better and for worse, didn't and couldn't do that.
As you say, the journalists were definitely part of that. To the extent a global conversation existed previously, it was among journalists and the people they covered, like major politicians, NGO heads, and the like. Twitter expanded and disintermediated that. Suddenly you could hear from all those media subjects directly. And even more unusually, you could try to join in, speaking directly to people previously unreachable, maybe even getting replies. One major motivator for people joining Twitter was exactly that, as the Waiting for Bieber art piece demonstrated: https://www.trendhunter.com/trends/waiting-for-bieber
The only platform with anything like that reach was email. And although email has global reach, it isn't a global stage like Twitter was. No other platform has that, and I doubt one ever will.
Did any other platform ever get so much attention from mainstream media?
My feeling as an outsider is that Twitter was useful and valuable ("high engagement") for the folks who enjoyed the culture that built up on the platform, but for everyone else it was always a bit insular and self-important. Since Twitter's founding, more and more online communities and spaces have sprung up, so if Twitter does indeed decline in usage its regulars will diffuse into the many other alternatives that have sprung up.
That's my $0.02 at least as someone who's watched from the sidelines for a couple decades.