Mark Zuckerberg is facing a second day of questions from US Senators over data breaches at Facebook.
Concerns over regulation and privacy dominated the first five-hour session.
One of the key moments was this exchange with Illinois Democratic Senator Dick Durbin.
"Mr. Zuckerberg would you be comfortable sharing with us the name of the hotel you stayed in last night?
Mark Zuckerberg, Facebook CEO: "Erm...er…no" (audience laughs)
Dick Durbin, US Democratic Senator (Illinois): "If you message to anybody this week would you share with us the names of the people you've messaged."
Mark Zuckerberg, Facebook CEO: "Senator, no, I would probably not choose to do that publicly here."
Dick Durbin, US Democratic Senator (Illinois): "I think that might be what this is all about, your right to privacy, the limits of your right to privacy and how much you give away in modern America in the name of quote 'connecting people around the world.'
Analysts say the social media guru, whom the world usually always sees in a t-shirt and jeans, grew in confidence as the hearing went on after initially appearing quite uncomfortable.
"He was very good at looking sincere the way he needs to, looking contrite, making eye contact when he needed to, looking down when he needed to look a little 'I'm hearing you' that sort of thing, so he certainly looked coached. He looked sincere. He looked like he wanted to work with the senators. But he also held his position," believes David Levine, law professor at University of California Hastings.
Zuckerberg's mea culpa also pleased the markets - Facebook shares ended the day on a 4.5 percent surge, the firm's biggest gain in two years.