In a lengthy "Why I quit" letter, former Google software developer James Whitaker blames Facebook envy for the company's decline:
Advertisers and publishers cherish [detailed] personal information, so much so that they are willing to put the Facebook brand before their own. Exhibit A: http://www.facebook.com/nike, a company with the power and clout of Nike putting their own brand after Facebook’s? No company has ever done that for Google and Google took it personally.
Larry Page himself assumed command to right this wrong. Social became state-owned, a corporate mandate called Google+. It was an ominous name invoking the feeling that Google alone wasn’t enough. Search had to be social. Android had to be social. You Tube, once joyous in their independence, had to be … well, you get the point. Even worse was that innovation had to be social. Ideas that failed to put Google+ at the center of the universe were a distraction.
Joshua Gans generalizes the lesson:
[T]he start-up turned into a corporation. It faced a threat that required a coordinated response and that response necessarily had to crimp the innovation system that had built it. The problem is that coordinated response had its own flaws. As I have argued previously, Google+ didn't offer consumers much more, if anything, than Facebook did.
(Chart from tumblr user The Asphyxiated Pecker)