ABC News' Mary Bruce Reports: With just 122 characters, President Obama has officially become the first president to tweet while in office. Although his 2008 victory proved his skill at using social media for political gain, it wasn't until this weekend that Obama finally sent his first tweet -- well, the first one that he's personally written at least. "Being a father is sometimes my hardest but always my most rewarding job. Happy Father’s Day to all the dads out there. -BO" the president tweeted Sunday. From now on, the president's initials attached to the end of any message on the @BarackObama account, will signify that the president wrote it himself, not a staffer, his campaign announced last week. There is even a new caveat posted at the top of the account, which is specifically devoted to the president's reelection and decorated with the Obama 2012 campaign logo. "This account is run by #Obama2012 campaign staff. Tweets from the President are signed -BO," it reads. The @whitehouse account continues to be for official White House tweets.