Children and young people feature in the terrorist group's propaganda, including a recent video showing boys in camouflage and ISIS bandanas learning hand-to-hand combat. The militants call them their "Cubs of the Caliphate."
Experts say ISIS has especially ramped up its efforts to lure young women -- seen as potential brides for its fighters -- into the territory under its brutal control in Syria and Iraq.
But that still leaves the question of why its recruitment drive appears to be proving so successful.
"There's no question what we're combating with ISIL's propaganda machine is something we have not seen before," U.S. State Department spokeswoman Jen Psaki told CNN. "It's something we need to do a lot more work on. We are seeing 90,000, I think, tweets a day that we're combating."
The militants have repeatedly promoted videos using high-level production techniques to celebrate their brutality. A short mash-up clip in September
played like a trailer for an action movie, with slow-motion explosions and flames engulfing American troops.
UK surveillance chief Robert Hannigan has said ISIS and other extremist groups use platforms like Twitter, Facebook and WhatsApp to reach their target audience in a language it understands. Their methods include exploiting popular hashtags to disseminate their message, he said.
ISIS also use its Western recruits to promote the cause to other people like them back home.
And the extremist group is putting a particular focus on girls, analysts say.
"We're seeing young women from across Western countries both expressing their support for and migrating to Syria now in totally unprecedented numbers," said Sasha Havlicek, chief executive of the Institute for Strategic Dialogue. "And I would say this is the result really of an extremely sophisticated propaganda recruitment machinery that's targeting young women very specifically."
It promises the good life
Analysts say that ISIS can appeal to young people's religious idealism and to a desire to escape the frustrations of life in the West.
"ISIS provides a utopian political project, the so-called caliphate, the centralized Islamic rule," Gerges said. "ISIS provides these deluded young men and women with an adventurous trip."
As a reward for allegiance, ISIS loyalists receive gifts from Allah including "a house with free electricity and water provided to you due to the Khilafah (the caliphate or state) and no rent included," according to Aqsa Mahmood
, a British teenager who left Scotland to join ISIS in 2013.
"Sounds great, right?" writes Mahmood, who now appears to blog about life under ISIS rule and is believed to have communicated with one of the three British girls who recently left for Syria.
Mahmood offers assurances to women who might be worried about roughing it: "You can find shampoos, soaps and other female necessities here, so do not stress if you think you will be experiencing some cavewoman life here."
Western officials, though, say that ISIS is pushing a false narrative of what it's really like in its territory.
People who heed its call "erroneously believe they are going out to help people," said Yasmin Qureshi, a British lawmaker. "And they don't think ISIS is doing anything wrong because they think ISIS is helping people in their sort of warped thinking."
It has a diffuse but efficient recruiting network
"It took so little time for these three young, innocent, vulnerable girls to from the heart of London to the heart of Turkey -- and probably they are in areas controlled either by ISIS or other militant groups," said Gerges, referring to the British teenagers.
He stressed "the important presence of a person who basically serves as a middleman or middlewoman between young men and women in the West and ISIS militants in Iraq and Syria."
U.S. investigators studied the case of three teenage Colorado girls
who tried to join ISIS last year to learn more about how the militant group recruits young people in the West.
The FBI found examples of Westerners already in the ISIS fold who were communicating directly with new recruits via social media.
Believed to be operating from Turkey, Syria and Iraq, the online recruiters helped provide how-to guides for Westerners who are inclined to travel and join the ISIS fight, officials said.
But even after identifying them, officials couldn't see a clear path to arresting them. Most are thought to be out of reach of U.S. law enforcement.
It's outwitting Western governments' efforts
Social media services may have been largely invented in the West, but ISIS is outflanking governments in how it uses the platforms.
"We are way behind. They are far superior and advanced than we are when it comes to new media technologies," Maajid Nawaz, a former jihadi and author of "Radical: My Journey out of Islamist Extremism," told CNN last year.
British Prime Minister David Cameron has said the case of the three British girls highlights the need to fight ISIS on multiple fronts.
"It needs every school, every university, every college, every community to recognize they have a role to play, we all have a role to play in stopping people from having their minds poisoned by this appalling death cult," he said.
The U.S. government is trying to step up its efforts to counter ISIS propaganda by beefing up a small State Department agency
to make it the heart of the fight against the militants' messaging.
"We're seeing their approaches continue to evolve," Psaki told CNN. "We need to continue to make sure ours are evolving and we're combating it in the most strategic way and using every interagency resource."
But she acknowledged that the United States still has a lot to do.
"We're really going to pick it up now," Psaki said. "We have new people in charge of the office. And we will see what happens over the coming months."