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.
Don't forget the steam queue. It use prediction to show games that correspond to your taste. It's not very accurate, but it allow to show new games on each iteration. You do not waste your time on the always same top 100 games. It allow to discover some strange or underated titles .
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.