Facebook users post more than 2.5 billion messages and updates each day, worldwide. All posted content must comply with the company's standards, which ban many forms of speech that, in the United States, are protected offline. iStockphoto.com hide caption

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Egyptians use their mobile phones to record celebrations in Cairo's Tahrir Square, the epicenter of the popular revolt that drove Hosni Mubarak from power in 2011. Twitter was often used to record happenings during the Arab Spring. Mohammed Abed/AFP/Getty Images hide caption

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