HN isn't technically social media, but it's close. The little vote buttons, the karma, the constant feed of "hot" articles, are designed to fuel the dopamine push that creates addictive engagement.

What it lacks to be true "social media" is engagement around a social network. There are certainly "social networks" at work on HN, but most of the users don't know each other and aren't focused on each other's statuses, pictures, videos, relationship statuses. Instead they're mostly focused on intellectual competition via comments. (when you're not allowed to compete for jokes, like folks do on Reddit, you compete for whatever else defines your identity, which on HN is intelligence).

Ironically, HN's always wanted to be a social network, in that they want to build "a community of users" where the users all recognize each other (they claim that's necessary in order to build a community). Once HN introduces personal blogs or tweet-like status updates, it will 100% be a social network, with all the same health concerns.