> After all, we're on social media here, right now [...] Similarly, Reddit is social media as well

I don't agree, but i can't really formulate why. For example is 4chan social media, or Something Awful? I'd say no, they're forums. How is that distinct from Twitter or facebook? Fundamentally the model feels completely different to me. The idea of user centric feeds/content doesn't really fit. Maybe it's that the content is not the OP but the latest days worth of posts?

I see how on that metric HN is not forum-y but social media-y. But there it's the user vs content centric aspect?

Where does Pinterest sit?

Hummm, not at all sure.