In a nationally televised news conference at about 11:40 pm EST on May 1, President Barack Obama announced Osama bin Laden had been killed at the hands of the US military. But unofficially the news broke more than an hour before, on Twitter, with the likes of Keith Urbahn, chief of staff for Donald Rumsfeld, saying they had heard the al-Qaeda leader had been killed.

Twitter also proved a key communication channel for police departments in some major US cities, in ...