This is actually a really good "bright line" distinction between something being a forum and something being "social media". On any site (forum, S.M. or otherwise), comments or articles must necessarily be ranked top-to-bottom, where top usually is the most visible. How this ranking happens often is the main driver of what the site is like.

- Chronological (either first on top or last on top): Not social media

- Site-moderator curated: Not social media

- User-voted: Social media

- Algorithmic (usually based on some opaque measurement of engagement): Social media

> User-voted: Social media

I don't think user voting automatically makes something social media. I think there's a blurry line between voting/"likes" and user-driven moderation.

User-driven moderation can certainly develop aspects of a popularity contest like social media has, but often it looks like a somewhat hopefully objective assessment of the quality of a comment, regardless of whether or not the moderator agrees with the point the commenter is making.

I'm not saying that HN's user moderation is fully objective. It certainly isn't; I don't fully moderate that way, myself, even. But HN's user moderation is absolutely not the same as the liking done on a Facebook post or its comments.

A funny thing; the Ars Technica front page sorta feels like social media—they don’t quite algorithmically rank the comments, but they do hide very downvoted comments (as a sort of community moderation feature), and sometimes the author of an article will highlight your comment (making it visible on the article page). I would call it exactly the dividing line between social media and not social media, to the point where I’m not sure which side it falls on.

Meanwhile their forum feels more like an old school PhpBB thing. Actually I think it was PhpBB at one point, until a recent redesign.

Now their front page comment section and their forum have exactly the same back-end. (Like if you leave a comment on their front page, it also goes into a thread in their forum). Largely overlapping community. But, the vibe in the two different areas is completely different.