I have been hating Quora all along for this, and wondered whether this constitutes cloaking (because they serve different content to Googlebot and the user), which is against Google's TOS.

The following are excerpts from Google's First Click Free (FCF) program page[1]:

    If you offer subscription-based access to your website content, or if
    users must register to access your content, then search engines 
    cannot access some of your site's most relevant, valuable content.

    Implementing Google's First Click Free (FCF) for your content allows 
    you to include your restricted content in Google's main search index.

Apparently Quora's answer to this is: "Wrong! We don't need to implement FCF to get our restricted content into Google, we simply cloak!"

    To implement First Click Free, you need to allow all users who find 
    a document on your site via Google search to see the full text of 
    that document, even if they have not registered or subscribed to see 
    that content. The user's first click to your content area is free.

So clearly the program applies in Quora's situation, yet Quora doesn't follow the program's guidelines to show "the full text of that document". I see this as strong evidence that Quora's actions violate Google's TOS, because otherwise what's the point of the FCF program?

[1]: http://support.google.com/webmasters/bin/answer.py?hl=en&...