NEW YORK – Facebook CEO Mark Zuckerberg often came across as one of the smartest people in the room as he jousted with U.S. lawmakers demanding to know how and why his company peers into the lives of its 2.2 billion users. But while some questions were elementary, others left Zuckerberg unable to offer clear explanations or specific answers. A series of tough inquiries about how much personal information Facebook vacuums up on and off its social network seemed particularly vexing for Zuckerberg, who couldn't quantify it.