GOP Sen. John McCain greets well-wishers as he arrives to visit a pro-European Union rally in Independence Square in Kiev, Ukraine, on Dec. 15. Strong criticism and trips like this landed the Arizona senator on Russia's travel blacklist. Sergei Chuzavkov/ASSOCIATED PRESS hide caption

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Jan. 15, 2009: As the U.S. Airways jet they had been on sinks into the Hudson River, passengers are rowed away. This isn't the iconic (and now copyrighted) photo that helped transform Twitter. But it does give a sense of what it was like that day, 5 years ago. Bebeto Matthews/AP hide caption

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Twitter made its debut on the New York Stock Exchange in November. Both the social media giant and the relative newcomer Snapchat are valued in the billions, but neither company has yet turned a profit. Emmanuel Dunand/AFP/Getty Images hide caption

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"DOUGH a crust, an unbaked crust ...": Carrie Underwood may have played Maria in NBC's The Sound of Music Live, but on Twitter, it was @DiGiorno that stole the show. NBC/NBCU Photo Bank via Getty Images hide caption

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A worker unveils a floor mat bearing the logo of Twitter on the floor of the New York Stock Exchange. Andrew Burton/Getty Images hide caption

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Twitter announced that it has set a price range for its initial public offering between $17 and $20 per share and hopes to sell 70 million shares. Justin Sullivan/Getty Images hide caption

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