The bigger thing is not what fraction of Quora users use facebook logins, but how much of Quora's traffic is from search results by non-logged-in users (who may be new to the site).
I think Quora would be fine with 20-30k active users with accounts, creating questions/answers/comments. They've rolled out features (Credits, and requiring Credits to promote questions) which imply having a large number of people asking poor questions and giving poor answers is something they're actively trying to avoid.
Yet, if someone like Keith Rabois or Harj Taggar answers something within his field of expertise on Quora, it gets great search placement, AND often ends up syndicated to Forbes, etc.
"Quora as a replacement for blogging, by high value individuals" is the thing to look out for, not huge daily active user counts. With great content, search traffic will follow, and then there will be some way to monetize.
Not every social network has to be peer to peer. If I got to watch a community of great physics experts (Quora actually has some very strong engineering expertise in mech/nuclear, surprisingly) interact, and then pay to ask specific questions, I'd be quite happy. Either pay cash, or pay with answering IT, banking, war zone, etc. questions asked by those physics experts.