Usenet/bbs/mailing lists are social media. Just one kind. Social media has been around for a while, it just got really big 10 years or so ago with the microblogging platforms like Twitter and Facebook.
HN is social media. Maybe niche, but still part of the wad.
We have personas and profiles and comment on stories. Pretty social.
Social adds the graph, etc. We don't have a "social graph", or friends and followers on HN (except implicitly).
We had online forums for a full decade (more if you count Usenet/BBS/mailing lists) before we had "social media".