A WebmasterWorld thread has discussion around what possible user metrics could Google be using for search ranking purposes?
Of course, Google says they do not use Google Analytics or Google Chrome data. So are they using Google+ and Google +1 data for rankings? Google's Matt Cutts said they don't really use much social data and has downplayed Google+ in the rankings to almost not being used at all in rankings.
What about bounce rates from the web page and back to the search results page? Google may be using that but in 2008 said that signal was spammable and noisy.
Could they be using data from their display ad network? I guess, but seems against Google's core principles to do that.
Google they be using ISP data? I guess so, but if so, why not just use Chrome data, which they said they do not use.
Most people think Google uses click through rate, CTR, for rankings. They store it and show it in Webmaster Tools, like they do with +1s (for now) and so maybe they use it. Of course, that is just as easy to spam as bounce rates.
What user metrics do you think they use?
Forum discussion at WebmasterWorld.