I'm no lawyer, but I think Quora would be required to credit me by copyright law? Because I haven't given them explicit permission to reproduce the material without crediting me. The agreement doesn't have a clause saying they can reproduce my content and pass it off as their own. I think they would surely need to have an explicit clause to do that.
And - if they don't say who created the content, then they would be passing it off as their own, would they not? I don't think it would be enough to just say "quora users" without saying which or at least linking to a page where you can find out.
For answers reproduced elsewhere, it's the same.Though they don't require people to attribute the content if reproduced elsewhere, they do say anyone who uses the service has to:
" attribute Quora by name in readable text and with a human and machine-followable link (an HTML <a> anchor tag) linking back to the page displaying the original source of the content on http://quora.com on every page that contains Quora content; "
The page that it links to would then tell them who wrote it. So other users also couldn't pass it off as their own.
In practice anyway they attribute everything AFAIK.