As Facebook heads for its 100 billion dollar flotation, Emily Maitlis updates her recent documentary on the prospects for Mark Zuckerberg's social network phenomenon. She examines how Facebook, now with 900 million users, plans to earn the billions its new investors will expect from it. With exclusive access to Mark Zuckerberg and senior executives, Emily tells the Facebook story, and reports on its challenge - to build its advertising business from the personal information its users provide, without losing their trust.