hn is largely a technology oriented link aggregator with discussions, and probably some would also classify it as a forum. Or as social news site as goes on wikipedia among fark, slashdot and reddit. But beside a voting system, simple profiles there's nothing else - this is nearly an experience unlike anything large social network services offer.
A typical social media platform mainly exists around main stream/feed, sharing content and building profile or groups dedicated to particular topics or around known brands. That's of course the perfect unstained image because everything falls apart when we start getting into the details, such as algorithms in the work, content quality and moderation and so on.
When MySpace came out, the profile was the home page for a lot of people, and the content orbited around that. Coupled with the mass movement to represent oneself faithfully online as in the real world, (maybe for banking, maybe for surveillance), I think social media sort of operates as a trap. On facebook, you are encouraged to upload your real photos of drunken night out, family vacation, or whatever IDs you in life. On LinkedIn this is mandatory, your "avatar" must mirror your physical self. I have a lot to say on this, but I think I'll just leave it at topic vs profile.
I do personally think the karma thing is an aspect , because it's gamed everywhere to huge advantage -- but the altruistic view is that its a branch of moderation, an effort to democratize the removal of obviously bad actors while still facilitating dissenting or contrary speech.
I also know that's a naive view.