In 2007, the founder of Facebook, Mark Zuckerberg, knew that he needed help. His social-network site was growing fast, but, at the age of twenty-three, he felt ill-equipped to run it. That December, he went to a Christmas party at the home of Dan Rosensweig, a Silicon Valley executive, and as he approached the house he saw someone who had been mentioned as a possible partner, Sheryl Sandberg, Google's thirty-eight-year-old vice-president for global online sales and operations. Zuckerberg hadn't called her before (why would someone who managed four thousand employees want to leave for a company that had barely any revenue?), but he went up and introduced himself. "We talked for probably an hour by the door," Zuckerberg recalls.

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