I guess HN and similar platforms could fit into the social media box, broadly. Taken extremely literally, they are mediated social environments. But I would absolutely make a distinction between sites like HN and "real deal" social media.

Social media companies like Facebook and Twitter measure success by things like user engagement and time spent on the platform - and their design principles are optimized to max out those measurements. That's how they create more value for advertisers and make billions of dollars.

This site is a social experiment in creating civil online discussion. Facebook and co. are a social experiment in hacking human behavior to create compulsive users who consume advertising with (measurable, trackable, hackable) enthusiasm.

I don't think HN would get much buzz for an IPO. :)