Mark Zuckerberg, the Facebook chief executive, began two days of marathon hearings in Washington, answering tough questions on the company’s mishandling of data. But the hours of testimony about the social media company’s practices seemed to focus on a larger, more difficult question: What is Facebook, exactly? Guest: Kevin Roose, who writes about technology for The New York Times. For more information on today’s episode, visit nytimes.com/thedaily.