I’ve been saying this was inevitable since (before) 2010, as the stars have been aligning for this to happen for quite some time.
From my 2010 post:
It seems to be an inevitability that all of this intelligence will one day soon be applied to power a socially-targeted ad network as big (or bigger than) Google’s AdSense. It would be a network that would theoretically deliver even better results for advertisers, resulting in higher CPMs/CPCs/CP-whatevers that can deliver higher payouts to publishers, making a choice between the two platforms a not-too difficult one for those publishers.
For everyone that bought into Facebook stock, this is a very big reason to believe.