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.
That is right. Norvig doesn't see what Facebook is good at and ask how it will make money. He turns the question on its head, looks to a successful business model and asks how Facebook could be used for it. And it is an uphill battle for Facebook to monetize in this fashion.
I think what we've seen so far is that Facebook is about creating social spaces, not solving problems, so it really can't be used that effectively as a recommendation engine. Facebook doesn't have a great way to seek rent on these social spaces because they've built their user base on free service. So where is the money going to come from? Probably selling user data to corporations.
