Facebook, thus far, has never ever been about being deep. The whole point is to cover the breadth of social interest – to surface the little bits of who you are, to as many people possible, so that you'll have some reason to chat with each other at the next barbecue.
It's about eliminating the search for potential like minds, or at least making it vastly easier to do so using (overly?)simplistic measures – shared interests, friends in common, and perhaps to stretch a bit, similar styles and topic of commentary.
Quora, on the other hand, is a lot more about thorough understanding of a topic, and/or intense desire to learn about it. Functionally, this carries a huge overlap with Facebook in terms of potential to surface friendships from the ether, but it is successful precisely because such discovery is based on the interest, rather than the social graph.
To answer your followups – I think Facebook could go a lot deeper as an app itself, but I feel pretty strongly that they've ceded "depth" to apps developed via their API, and want to continue to focus on the much looser flow aspects of "surface bonding" – that is, to choose speed (especially in terms of viral expansion of Facebook the platform) over strength. Because, once everyone on Earth is a Facebook member, there's an innate strength that's built simply in terms of network effect defensibility versus competing products and platforms, along with a fluidity of form and function that could allow FB to gain adoption in widely disparate countries and cultures in the manner that suits these societies best.
Simply put, it's obvious to Facebook that their products are lacking, but they choose, for now, to ignore some of the aesthetic and intellectual deficiencies precisely because doing so creates a wider, more all-encompassing platform that more people/organizations will choose to adopt.
Quora, for its part, goes so far as to specifically call out the fact that it is pretty much "English only" for now in various parts of the interface – an indication that they're intent on creating a much more robust, deep solution to a smaller set of problems, before worrying too much about how to conquer the Chinese market.