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Mark Zuckerberg, the Facebook chief executive, began two days of marathon hearings in Washington, answering tough questions on the company\u2019s mishandling of data. But the hours of testimony about the social media company\u2019s 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\u2019s episode, visit nytimes.com/thedaily.