When Twitter could take credit for revolutionary political movements like the Arab Spring, it was easy for the company's executives to joke about their liberal stance on free speech. (Twitter, they said, was "the free speech wing of the free speech party.") But things are a bit more complicated now, as Twitter increasingly plays host to bullies, harassers, Nazis, propaganda-spreading bots, ISIS recruiters, and threats of nuclear war.

Get in-depth coverage of current and future trends in technology, and how they are shaping business, entertainment, communications, science, politics, and society.