The site-wide quality of answers matters less for Quora to be useful to an individual when you're following people rather than topics. You see answers upvoted by people you follow or answered by people you follow.
Quora will continue to work for the same reasons that Twitter does...I get tweets from people I follow, millions of bieber fans don't affect my twitter homepage.
Quora will continue to work for the same reasons that Twitter does...I get tweets from people I follow, millions of bieber fans don't affect my twitter homepage.