The active Quora userbase loves the product and are not at all looking for somebody to build something better - that is usually a prerequisite to an incumbent getting beat.
Quora is a tiny speck in the market compared to the number of Internet users who want informative answers to questions. Quora's user interface is a giant fail, as it has already convinced many of the most knowledgeable people in whole disciplines not to post content there, and not even to look for other people's content there.
