For security purposes — and perhaps also by preference? — the President of the United States of America uses a BlackBerry. This created some confusion today when Barack Obama snapped up the @POTUS Twitter account and some savvy users discovered that he was tweeting from an iPhone.
This nearly devolved into a national scandal, but both The Washington Post and BuzzFeed have determined that the device belonged generally to the Executive Office of the President, not to the president himself. (It is unclear if the president's "secure" phone has, or is able to use, Twitter.)
When you stop and consider the account — @POTUS — more closely, maybe this situation makes sense. Obama won't be @POTUS forever. One day soon, when he is no longer president, the account will be transferred to the next duly elected leader of the country, in a transition of social media power laid out by the 22nd Amendment to the Constitution.