I'm not sure if "subreddits" / "interests" alone make reddit social media, but combined with its scale it definitely does.

I have blocked hundreds of subreddits just to make my reddit experience tolerable.

I think there's a few well-moderated large subreddits that I read periodically that are pretty similar to HN. Like if you go to r/linux, I doubt most people would say that's toxic. It's probably worse than HN, but that might be down to dang be good at moderating.

I guess it's a social medium but it feels different.