A main difference between {MySpace, Facebook, Twitter, Instagram, etc.} and {reddit, hn, chronological forums, etc.} is that the former heavily rely on following/friending (i.e, establishing your social network) while the latter do not. Thus, the latter are not "social media" in the sense of media existing in a social network.
Also, I find that when replying to something in the former I'm thinking about the person (I know them, in some sense) and what they said, while in the latter I'm thinking almost exclusively about what they said (I probably don't know the person). The latter lacks that social element.
If every forum qualifies, you're using the term so broadly that it's useless. That's what everyone else understands, which is making it seem to you like they're missing something you see. They're not.
For comments that's mostly a consequence of a tree-based commenting model, whose huge benefits over linear model more than offset any "damage" caused by it not being strictly chronological.
I am tempted to think the amount I have used social media and my smartphone has actually declined since 2014, and in the case of facebook I suppose it considerably has, but I'm not sure I haven't just replaced it with other compulsive clicks. So in that sense I must agree things have probably escalated a bit, but I can't point to a single event over the past decade to create a turning point.