If HN is social media, then so are PHPBB, NNTP, BBS, etc. and the term loses its semantic relevance.
My heuristic is that social media focuses on particular people, regardless of what they're talking about. In contrast, forums (like HN) focus on a particular topic, regardless of who's talking about it.
Doesn't matter what you want it to mean. What matters is what those in power want it to mean. It's very easy to stretch the definition to cover all sites where people can post content for strangers to see, or stretch it even wider to all digital media where people can interact with a social group.
AFAIK nobody here is. The point is that with relevance to the current discussion on potential future age-verification laws, only the widest definition matters, because that's what's at risk.
My heuristic is that social media focuses on particular people, regardless of what they're talking about. In contrast, forums (like HN) focus on a particular topic, regardless of who's talking about it.