It was a Wednesday night in San Francisco's SoMa neighborhood, and Jared Cohen, the youngest member of the State Department's policy planning staff, and Alec Ross, the first senior adviser for innovation to the secretary of state, were taking their tweeting very seriously. Cohen had spent the day in transit from D.C.; Ross hadn't eaten anything besides a morning muffin. Yet they were in the mood to share, and dinner could wait. It wasn't every day they got to tweet about visiting the headquarters of Twitter.