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WASHINGTON - JULY 09: Presidential candidate Senator Barack Obama and Senator Hillary Rodham Clinton work their cellphones and chat together on his campaign plane at Reagan National airport  in Washington, DC, July 9, 2008. The two are traveling to New York together for a series of fundraisers. (Photo by Charles Ommanney/Getty Images)
WASHINGTON - JULY 09: Presidential candidate Senator Barack Obama and Senator Hillary Rodham Clinton work their cellphones and chat together on his campaign plane at Reagan National airport  in Washington, DC, July 9, 2008. The two are traveling to New York together for a series of fundraisers. (Photo by Charles Ommanney/Getty Images)

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From Twitter town halls to the Pope's first tweet, CNN's Atika Shubert looks back at the major milestones of Twitter.