"This will require them going from a niche advertiser to taking over a significant fraction of all worldwide ad revenue, this is no small feat."
But it is small for Facebook. They already have the footprint with over 9 million sites all running the Like button. Utilizing that same JS they can have a 'Social Adsense' revenue stream overnight. They can potentially grab the search queries from the headers and have implicit and explicit data to target off of (the holy grail of targeting).