The marriage of Mark Zuckerberg to Priscilla Chan a day after his Facebook stock offering closed an iconic chapter in American business history. The May 18 initial public offering of Facebook (FB) clinched Zuckerberg's status as a billionaire whiz kid who parlayed a dorm room hobby into one of the world's best-known companies. But with the IPO launched and champagne glasses put away, CEO Zuckerberg now faces his biggest challenge: