I wonder how accurate this is.
I have no account in any social media except in LinkedIn (which I see more as a job portal than social media), and that's how I raise my kids. I live without facebook, instagram, twitter, or whatever else and I'd risk to say life is better without those.
"For instance, news stories that express outgroup animosity are 67% more likely to be shared on social media (Rathje et al., 2021). "
They are holding an extreme view and posting it online.
Personally, and this is a bit of a "hot take", but I think some people trying to define a difference is just trying to justify continue using HN and forums like Reddit while still hating on social media or wanting it banned. To be consistent with getting off social media, I think one ought to get off HN and Reddit completely as well (use a site blocker extension). But yeah this is a bit of an inflammatory opinion, I understand.
That being said, voting mechanisms which change the order of user-contributed content is hallmark of social media shared by Hacker News. I like to think, though, that this place has a strong enough community to prevent the voting dynamics from devolving to what you see on the most politicized subreddits. (I'll mention that the quality on here is in no small part the result of dang's diligent moderation, btw!)
An interesting message board to compare Hacker News with in this regard is actually 4chan. The former is maybe less social media like inasmuch as we have no inline images here, so there is no room for the sort of meme culture you see on the latter. On the other hand, 4chan is entirely bereft of voting (and even accounts to assign karma to, although reusable tripcodes come the closest), and as such is in some ways even more egalitarian (in contrast to non-anonymous social media).