I wonder how you would value sites like Rap Genius this way. It is pretty much an SEO play, so I would expect only a very small fraction of users to come back.
it's risky to build the core of your business on someone else's platform. You could argue Quora is doing something similar and I'm just as confused by them...
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.