Correct me if I'm wrong, but:
1. Third party statistics sites like Alexa and Compete are horribly untrustworthy indicators of reality.
2. Growing traffic isn't the best way to foster a community. I believe pg has written about this a few times, with regards to communities in general and HN.
I think the Quora community is amazing. It's very populous and the quality of answers is generally on par or better than specialty interest sites/forums from a wide range of topics, all in one place. Of course, it has its strengths and its weaknesses - for example, a strength would definitely be reading an answer from an actual CIA [something] analyst, former Foreign Service Officer, doctor, etc. on a relevant question, but it falls behind in areas like technology IMO.
The site is annoying and I hate a lot about the community (surprise, it turns out "mature adults" can use shitty memes just as much as the teens everyone blames for them on reddit), but I think they've done a great job at fostering the kind of community they want so far. If anything, it's being threatened by the pressure of growing.