There is no network or linkage between users? And people are not allowed to post inane topics or messages.

Pretty basic.

How do you think the voting ring detection works?

Some users post contact info in their profile, so there is the possibility for messaging (email). Yes, there are policies, and structure and many good things - I'm not sure that means that HN isn't a social network of sorts?

I suppose it might boil down to a technicality, but I always read "social network" as a graph that describes how a group is connected - not as some strict technological definition. So a bunch of people that informally meet at a cafe to discuss politics, art and other topics can be described as a social network.

That is an implementation detail not a feature.

each human can have multiple identities as well.