This number is based on users logging in through FB connect only. From the Techcrunch story:
"According to AppData, 20K daily active users and 180K monthly active users log into Quora through Facebook Connect — Bear in mind that this is a small fraction of its total number of users, which Cheever and D’Angelo famously never reveal."
Yes, it does seem kind of low when you look at the activity compared to other sites like Stack Exchange sites. However, something I've noticed is the higher percentage of well known people answering questions on Quora. I could see them making a play for increased participation among well connected or high expertise individuals and focusing more on the quality of content rather than quantity. It would certainly be an interesting avenue to exploit should they choose to.
That seems rather...low for a $400 million valuation.