In the spring of 2014, an American hip hop producer unwittingly triggered an online hurricane with a quickly thumbed tweet featuring a four-word demand: #BringBackOurGirls. The hashtag called for the release of 276 Nigerian schoolgirls who'd been kidnapped by a little-known Islamic terrorist sect called Boko Haram. Within hours, the campaign had been joined by millions, including some of the world's most recognizable people: Oprah Winfrey, Pope Francis, David Cameron, Kim Kardashian and Michelle Obama.