Facebook was, while mostly not a public place (at least the way I used it), was always pretty gregarious by their design, in my experience. I admit to this being public and I believe I see how you are making the distinction. I accept it as a valid one to make.
On the other hand, while Facebook and the like had me interacting with those in my relational proximity, here it seems I am relating with people more in my cultural and professional proximity (largely due to the rules and cultural norms here). Sometimes it's just small talk and at others more technical or serious life matters but it's communicating with other humans (and likely a few bots) about life, sharing information and ideas, challenging one another to learn and grow. These seem like deeply social behaviors and that's a big part of why I regard this as social media.
With regard to the mall, that seems far more like Twitter or Reddit than here. While Reddit is less clear, Twitter is widely regarded as social media. On the other end is email that is "a human talking to another human" and regarded as not social media. Not to say you're wrong, just trying to explain my view and offer that as a bit of evidence that the boundaries are not so clear.
