社會運動
By 2009, members argued that it was "by far the largest socialist organisation in the United States today, attracting to revolutionary ideas a much larger number of young activists than any of the others." Four years later, an outside observer estimated that the organization had "at least 1500 members." The ISO also helped to organize the Chicago Teachers Union strike of 2012, which it characterized as an example of a new era of Social movement unionism.