from wiki:
In 2009, marketing staffer Peter Costanzo suggested to Trump that he could use social media to draw attention to his book, Think Like a Champion, which was due to be released later that year. He was unable to use the username @DonaldTrump, as it was already being used by a parody account. He and his marketing team decided to use the username @realDonaldTrump. Trump joined Twitter in March 2009 and sent out his first tweet on May 4, 2009