Facebook, Twitter, Instagram, etc, are primarily networks of personal (or corporate) identities. The centerpiece is the user. HN and Reddit are primarily ranked lists of interesting URLs. I don’t know one user from another, apart from a handful of prolific users, and half the accounts start with “throwaway”. (It wasn’t so in the beginning, but even then there was no reason to go to someone’s user page.)

I think you are misunderstanding my use of the the phrase "identity-based". What I mean is that the primary entity around which conversation and content revolve on HN is the topic (from URLs or questions), not the individual user's identity.

Topic based forums are a thing. I'm a member of a paid (gasp!) forum for high-performance building methods.