I think Norvig looks at Facebook as a lost opportunity for data mining: Facebook isn't used as a recommendation engine any more than Google is used as an ad company, yet all user actions always gets piped back to ad results. If John is friends with Sally (who is kind of cute, and kind of a photography geek, and recently bought a good midrange point-and-shoot), John subconsciously wants that brand to show up somewhere in the recommendations, even if it is too expensive. But Google doesn't know that John's friends have bought that camera, because who posts what camera they bought on their Google profile? If Facebook has the ad instead of Google, then Facebook might get the all-important click.