I really like how Andy Baio describes Quora’s practices as “isolationist” in this post about why you should never use their service:
All of Quora’s value is derived from the answers provided by its users, and they go to great lengths to make a well-designed platform for finding and replying to questions.
But they do everything they can to make sure you can’t get those contributions back out.
[...] When Quora shuts down, and it will eventually shut down one day, all of that collected knowledge will be lost unless they change their isolationist ethos.
I’ve never used Quora and I feel like the issues that Andy outlines is why. The same thing applies to Pinterest as well — although I’m not sure about their API or content policies — whenever I’ve stumbled into Pinterest I feel like it’s a service on the web, but not of the web.