My reasoning is that the socialness here doesn't affect my experience in any way (that I'm aware of). "Liking" stuff by upvoting doesn't mean _I_ get more of "Content X" on my frontpage, it just means everyone gets more of this content. And I can't friend/follow people so my frontpage can't be affected by a secret algorithm that shows me more content of what my clique is currently raging about.

So I both agree with you and disagree with you. Our terminology is lacking for sure. And I agree that this is _more_ of a discussion forum, but not just simply that. But I wouldn't classify a discussion forum as a subcategory of social media. Maybe the other way around.