When I visit Quora, mainly through it being a result for something I've searched for on Google, I can never quite understand what's the question, the answer, and what's an answer to something completely different. So I've never felt compelled to stay or join myself. Also, for a while I just hit a login wall, which of course makes you not click a link there the next time.
I’m not sure what their business model is, I assume ads, so I always wonder what good the login wall does and making the UX so horrible. I too only ever get there from a Google search too, much like Wikipedia, but with Wikipedia I’m usually going to make a few clicks once I’m there as the related link exploration is natural. I assume quora could get more clicks out of me if it wasn’t so awful. They’ve instead taught…
Probably also email harvesting and any associated segmentation they get with your reading history, since it's required to sign up to read anything.