It's a site that if you go to it you see only a login page allowing either Facebook or Twitter sign in, with no explanation of what it is or why you would want to give your Facebook or Twitter passwords to them or what they might do with them. (Yes, I understand how it really works but I am presenting how it looks to normal people.)

http://www.quora.com/

If you manage to find pages they expose to google to capture searches for information, you find it is like Yahoo Answers, except the answers are all blurred unless you agree to sign in and be tracked.

Not surprisingly, this user-hostile abomination of usability has commercially failed. For some bizarre reason the designer who created this nonsense is described as a usability design genius in the referenced article when it is clear from even a moment's glance at the site's usability that its designers are not fit to design a hole in the ground for tossing refuse.

It's looks like stack exchange but they hide the answers behind a facebook login.

Looks like junk. Not intuitive at all, forces a facebook/twitter login, huge turnoff.