But what if ISIS’s much-hyped social-media juggernaut isn't as important as all of these measures suggest?

“We know it has the potential to influence, but exactly how and at what levels are quite unknown,” Anthony Lemieux, an associate professor of communication at Georgia State University, wrote in an email. Lemieux is researching that very question, but in the meantime it’s difficult to find a reliable estimate of how many ISIS fighters have been radicalized and recruited primarily through social media. Max Abrahms, a political-science professor and terrorism specialist at Northeastern University, suspects the number is lower than many people believe. “There are other groups”—such as Boko Haram in Nigeria—“that have rapidly expanded their membership size in the absence of social media,” he pointed out to me. "Battlefield success is a better predictor" of group size than is social-media activity, Abrahms said. If, as some contend, ISIS's battlefield momentum has already stalled, its recruitment could suffer even as its social-media activity remains constant.

In tandem with its military successes, ISIS has also likely benefited from an influx of foreign fighters to Syria that predates the group's blitzkrieg in the summer of 2014. A record number of foreigners had already joined a variety of Syrian rebel groups by mid-2013, a full year before ISIS captured the Iraqi city of Mosul and began consolidating territory across the Syria-Iraq border. At the time, Thomas Hegghammer, a senior research fellow at the Norwegian Defence Research Establishment, acknowledged the role of social media in "the scale and speed of the mobilization." But, he continued, "this does not mean that social media in and of itself drives recruitment." Citing poorly policed borders and ease of travel to Syria, Hegghammer theorized: "The bottom line is that record numbers of foreign fighters are going to Syria because they can." Since then, ISIS's victories, among other factors, have enabled the organization to eclipse other rebel groups in terms of recruitment.

Western policymakers are quite reasonably preoccupied with ISIS’s recruitment of jihadists from Europe and the United States. But by far the biggest suppliers of the Islamic State's foreign fighters are Middle Eastern and North African countries, particularly Tunisia and Saudi Arabia, where broadband access lags behind access rates in the West. Among those who are online, according to a Soufan Group study of foreign fighters in Syria, potential recruits in the Levant and the Gulf "are interconnected within self-selected bubbles, and are isolated from anything outside." This implies both that social media helps ISIS amplify its message among closed groups that are already receptive to it, and that there are limits to how far that message can spread beyond those circles.