  Recent terrorist attacks carried out on behalf of ISIS on American and
European soil by lone wolf attackers or sleeper cells remind us of the
importance of understanding the dynamics of radicalization mediated by social
media communication channels. In this paper, we shed light on the social media
activity of a group of twenty-five thousand users whose association with ISIS
online radical propaganda has been manually verified. By using a computational
tool known as dynamical activity-connectivity maps, based on network and
temporal activity patterns, we investigate the dynamics of social influence
within ISIS supporters. We finally quantify the effectiveness of ISIS
propaganda by determining the adoption of extremist content in the general
population and draw a parallel between radical propaganda and epidemics
spreading, highlighting that information broadcasters and influential ISIS
supporters generate highly-infectious cascades of information contagion. Our
findings will help generate effective countermeasures to combat the group and
other forms of online extremism.
