Maybe? I guess it depends on how you define social media. It sure has elements of social media, with its up and down voting, and the karma scorecard. People here don’t tend to know each other though and there is no following, re-tweeting, or even lazy image sharing, so I don’t know, is HN social media?

IMO Hacker News is representative of the slightly earlier than social media concept User Generated Content:

https://en.wikipedia.org/wiki/User-generated_content

The original concept of Web 2.0.