John Kerry has officially become U.S. secretary of state, replacing Hillary Clinton as the top U.S. diplomat. Kerry, who was sworn in on Friday night by Supreme Court Justice Elena Kagan to the top job at the State Department, will address a giant staff of 70,000 running over 275 missions, the world's largest diplomatic network. President Barack Obama named Kerry, a former senator and 2004 Democratic presidential candidate, on Dec 21 to become America's 68th secretary of state.