HN is a form of social media, though: users submit content, comment on content and each other, and can upvote content and comments.
And there is "but there are people WRONG on the internet!" effect, which means demonstrating domain knowledge and winning arguments drive their interest -- something that seems strikingly more common with knowledge workers (e.g. lawyers, programmers, professors), than, say, construction workers. People wanna feel smart, esp. in their areas of proficiency and dominance.
I'm sure someone will post the relevant XKCD any minute now...
It also has no notion of relationships between users.