If you're extending the definition of social media to include sites like Hacker News, you may as well categorize the entire Internet as social media
I feel that the sine qua non of "social media" is user generated content conducted between specific users, tied to specific accounts.
Not even wikis meet that definition, and certainly not most of the author-generated content-based site like blogs or market sites such as amazon, or news sites where the discussion isn't the putative content.
I understand the point that literally any website could be thought of as "user generated"... I've personally made quite a few websites. But the level of interaction and the modalities of how I and a lot of folks use things isn't as interactive as, say, twitter.
So, perhaps for some user populations a Wiki is a social experience, especially for the folks who edit them. Though from my perspective, it feels entirely possible to use it without engaging in that social aspect.
I don't feel the same about Hackernews and/or reddit in general.
I feel like "user generated content conducted between specific users, tied to specific accounts, where users have one-to-one public conversations and those conversations are foregrounded as the site's content" is reasonable.
So, if that's my definition Wikis aren't social media and HN would be.
But I also feel like HN is just a flavor of Reddit, and I feel that most people understand Reddit to be "social media".
I could be worng about those feelings.
We decided we wanted to derive cars, but without the harmful effects of being in crashes, so we decided to create safer vehicles. I like some drugs, and a lot of them are okay in some situations with some boundaries... even if a lot of the users are using them for unhealthy reasons in situations that make the drugs problematic.
I like social media-- I'm literally using it right now to communicate with you. But I do that because I value your thought (even though I disagree with it) and I am able to interact with you in a space that both moderates my behavior by giving up/down vote feedback and which doesn't have a lot of tolerance for poor discourse habits.
If you apply those same rubrics to other social media and don't tolerate them when it violates those boundaries, I believe it's possible to use them in healthy ways.