I think the distinction is pretty easy imo. HN is topic centered, Social Media is person-centered. Before MySpace there was a pretty big proliferation of forums and other topic centered discourse. The profile was such a minor part of those tools.

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.