But those forums are under increasing strain. Al Qaeda's top-tier forum, Shamukh al-Islam, was down from Dec. 5, 2012, to Jan. 29, 2013, according to the study. That takedown, as well as two other major strikes last spring, left a void and accelerated migration to social media, which now is "beyond a point of no return," Mr. Zelin says.
Other researchers have registered similar observations. Evan Kohlmann, an expert on online jihadism, tweeted in December: "Due to the absence of top jihad chat forums, al-Shabab ... in Somalia has been forced to rely on Twitter to distribute its latest video release. This may be the first time that any terrorist group allied with Al-Qaida has ever used Twitter as the exclusive point of release for media."
New technology helps. New features on Twitter, Facebook, and YouTube make it far easier for global "jihadi entrepreneurs" to share articles, news, and videos.
"The newer technologies lowered the bar for participation, making the involvement of low-level or non-jihadis in the online conversation a new feature of the global jihadi movement," the study says. "Those so inclined can talk about jihad all day on the Web, even if they are geographically dispersed. This was not possible beforehand."
For now, forums still make up the core of Al Qaeda’s online presence because they can facilitate private conversations and are seen as authentic. (Social media sites and tweets are far easier to fake.) But social media have a growing role.
"Currently, the forums are the hubs where the al-Qaeda organization meets its grass-roots supporters in a relatively safe and exclusive environment," the study notes. "The social media platforms are where the product or ideas are sold."
What is unclear is whether Facebook would be more successful in selling the terrorist message than a forum. Statistics show that the English-language versions of jihadi websites are failing to spread the message to English-speaking Muslims in North America and Britain, the study says.