Aardvark's acquisition by Google for $50M about 18 months, back when the market wasn't as bubblicious as it is now, is one comparable. http://redmonk.com/sogrady/2010/02/12/aardvark-google/ looks at why that valuation arguably made sense. Aardvark had $6M invested, Quora's got $11M.

Like I said in my other comment, not sure what valuation makes sense, so I'm not convinced it'll be in the X00,000,000 range... but it's not impossible.