I barely use Steam as a social platform, and still I have a thousand times better experience when it comes to discoverability. I understand what you're saying — people spend more time using Steam than they do the app stores (and they also have access to social graphs and what-not, although you could argue Apple & Google have this as well via messages).

I would argue that point is kind of moot, since many of the ways I discover games on Steam would work without any friends or a social graph.

The sales (holiday, weekly and daily), curators, user tags etc, etc. They would all still work.

They just don't care.