The Pulitzer Prize-winning author of “A Problem From Hell”: America and the Age of Genocide, a fierce examination of American responses to mass atrocities in the twentieth century, serves as the country’s youngest-ever ambassador to the United Nations. She uses Twitter to defend human rights, highlight growing crises and post musings and pictures from her diplomatic duties. Since assuming her role last year, she has become one of the Obama administration’s most active and outspoken voices on the network. —Andrew Katz