Something that Facebook and Quora have in common is that the initial set of users were cool enough to prime the pump of viral growth. With Quora, you had awesome people giving answers only they had. Like a startup co-founder talking about the early days of their now successful venture.
So starting with awesome people creating awesome content helped. Over time, Quora has done a few things really well: (1) growing the community while keeping it fairly high brow, (2) their topic taxonomy, and (3) their email digests.
My problem with Quora is that it's more entertainment than useful. SE/SO is more functional. Who really cares what are the things about San Diego that most people don't know?