I think you've hit the nail on the head in many ways.

I was a Facebook addict until about two years ago. Many of my Facebook friends were wonderful people, but FB per se gives you a superficial window on relationships. Medium (as a social medium) gives you much greater opportunities for expanding camaraderie and dialogue. On FB or Twitter, you tend to share someone else's page / post / idea. On Medium, you're sharing your own in-depth response.