Microsoft CEO Steve Ballmer had flown to Palo Alto to visit his young counterpart twice. As Zuckerberg is wont to do, he took Ballmer on a long walk. Zuckerberg told Ballmer that Facebook was raising money at a $15 billion valuation. But Ballmer had come with something more sweeping in mind. "Why don't we just buy you for $15 billion?" he replied, according to a very knowledgeable source. Zuckerberg was unmoved even by this offer. "I don't want to sell the company unless I can keep control," said Zuckerberg, as he always did in such situations. Ballmer took this reply as a sort of challenge. He went back to Microsoft's headquarters and concocted a plan intended to acquire Facebook in stages over a period of years to enable Zuckerberg to keep calling the shots. But Zuckerberg rejected all the overtures. What Ballmer finally agreed to instead was an advertising deal that included a provision for Microsoft to pay a huge amount, $240 million, for a sliver of Facebook, 1.6%. Microsoft's investment gave Facebook an implied value of $15 billion.