Not sure it's the case with Quora. I'm just guessing here, but I would say they have high rates of direct visits and many reasons for users to keep coming back

Not sure it's the case with Quora. I'm just guessing here, but I would say they have high rates of direct visits and many reasons for users to keep coming back every day (or at least every week) because of quality of content and some of their gamification elements.