ISIS does have an enthusiastic base of supporters on English-language Twitter. But there’s a major difference between retweeting beheading videos or trolling the State Department and actually going to fight for the Islamic State. (For example, one vocal online propagandist for ISIS was revealed last December to be a corporate executive living in Bangalore, India.) According to official estimates cited in The New York Times, 150 people have “traveled, or tried to travel, to fight in Syria from the United States.” It’s not clear how many people actually succeeded in doing so. But it’s hard to imagine social media being the decisive factor in the decision to leave, say, suburban Colorado for Syrian battlefields, even if social media proved helpful in planning the trip. The causal relationship could just as easily work in the opposite direction: People engage with ISIS through social media because they’re already radicalized, rather than getting radicalized through social media.
Perhaps it’s the novelty of ISIS’s social-media operation, or its status as a major source of information about an otherwise largely opaque enemy and war zone, that make the group's tweets seem excessively threatening. The recent report that ISIS produces a large volume of social-media chatter—90,000 "tweets and other social media responses every day," by the Times's estimate, which may be conservative—doesn't actually say much about the role those communications play in terrorist recruitment.
As Brian Jenkins of the Rand Corporation noted in a 2011 study of "homegrown" extremists in the United States, reaching potential recruits online "does not mean radicalizing, and radicalizing does not mean recruitment to violent jihad." He also noted that while the Internet could "serve as a source of inspiration ... it may also become a substitute for action, allowing would-be terrorists to engage in vicarious terrorism while avoiding the risks of real action." Similarly, J.M. Berger, a Brookings analyst who studies extremists' use of social media, observed in 2011, "There is a tremendous amount of radical activity online. Very little of that activity will translate into real-world threats." Writing in The Atlantic, Berger also pointed out that even the volume of this activity may be less than meets the eye, since ISIS uses a number of techniques, including automated tweets and organized efforts to trend certain hashtags, to give the impression of a larger network of online support.
ISIS is surely a formidable force. Offline, the group is now estimated to be 30,000-strong or bigger. But the roots of its expansion probably don't lie on the virtual battlefield. More likely, they're on the real one.