Then wouldn't public access TV, call-in radio shows, or the news be social media as well since they typically involve many-to-many interactions? Or is there more to it?
Sure. It's probably more helpful to think of this (like almost all things) on a spectrum.
HN is definitely on the social media end of the spectrum. For example, all your listed examples have very hands-on and comprehensive, opinionated editorial control. HN has moderation, but not really editorial control per se.
