We are discussing with each other publicly on a website, sharing ideas and information. Each of us is doing so tied to an identity, such as it is. Just as we might on Reddit, facebook, instagram, etc. All are social media. HackerNews is by no means different.

In what way do you find it is not social media, or maybe I should ask how narrow is your definition of social media?

I tried to find how exactly DHS defines "social media" because that is actually what is relevant to this discussion, but I was unable. I think someone had posted their definition in one of the other threads on this topic, if you want to try to find it yourself.

Feel free to read through Wikipedia's article: https://en.wikipedia.org/wiki/Social_media

How is having an online discussion different if it's on Facebook or HN?

First, there's no real name policy here. Second, the quality of content and discussion is much higher here than on Facebook. Third, the HN community is tiny compared to Facebook. For starters.

It's a social voting website (not to mention the forum).