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.