Using sophisticated recruitment techniques, the jihadists have attracted hundreds of supporters from Western countries to the swathes of territory they have seized in Syria and Iraq.
In the beginning, these efforts focused on luring young men to join the ranks of jihadist fighters.
But IS is increasingly targeting young Western women, as dramatically illustrated last month when three British teenaged girls ran away from their London homes to join the jihadists.
Experts say many are shocked by what they find in IS territory, but by then it is too late.
"IS sells its Islamic utopia to these young men and women," said Lina Khatib, head of the Carnegie Endowment's Middle East Centre in Beirut.
"It tells them that this is the only real Islamic state in the world, and that they can become important figures in it."
Experts say the number of Westerners who have travelled to the "caliphate", which is more than twice the size of Jordan and home to about six million people, is about 3,000, including roughly 550 women.
In the latest high-profile case, three close friends from London -- a 16-year-old and two 15-year-old girls -- are believed to have crossed into Syria last month after flying to Turkey from London.
One of them is believed to have been in contact with Aqsa Mahmood, a woman from Glasgow, Scotland, who reportedly travelled to Syria last year to marry an IS fighter.
Mahmood's English-language blog -- "Diary of a Muhajirah" (Diary of a Traveller) -- has become a key source of IS propaganda aimed at women.
- 'Till martyrdom do us part' -
"We don't pay rent here. Houses are given for free. We pay neither electric nor water bills. We are given monthly groceries. Spaghetti, pasta, can foods, rice, eggs," Mahmood wrote in one recent post.
Work is available too, she said, for women who want jobs in education and health care.
Marriage in IS territory also has its perks, Mahmood has written.
IS fighters receive "seven days off" as part of the wedding celebrations and new brides can choose their own dowry -- but instead of requests for jewellery, they ask for Kalashnikovs.
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"Newly married couples are given $700 as a gift," Mahmood wrote. "We don't have fireworks but we celebrate the wedding by gunshots and lots of takbeer," or shouts of "Allahu akbar" (God is greatest).
A picture featured on her site of a bearded fighter and his new wife, dressed in a full white veil, was captioned: "Till martyrdom do us part."
Responding to a question from a young girl conflicted about leaving her boyfriend and moving to Syria, Mahmood wrote: "I promise, that one day someone will hug you so tightly and fix back all the broken pieces -- Yes, it's your halal husband."
The term halal is used to describe things -- from people to food and services -- that comply with Islamic religious requirements.
- 'An empty project' -
Other IS recruitment tools, such as its online English-language magazine Dabiq, also tout the benefits available to arriving foreigners.
"There are plenty of homes and resources to cover you and your family," the magazine has written. According to Iraqi officials, IS has redistributed to fighters any homes vacated by civilians fleeing the group's takeover.
In its own version of classified ads, Dabiq advertised that IS needs "judges, people with military or administrative skills, doctors, and engineers" to travel to its territory.
Carnegie's Khatib said it was clear that propagandists like Mahmood were working under the supervision of IS.
"If she's real, then there's no doubt that the Islamic State group supervises her," she said. "She's one of their recruitment tools."
"These girls join this organisation looking for adventure... and some of them live in an imaginary world, dreaming of marrying fighters," Hassan Hassan, author of "ISIS: Inside the Army of Terror", told AFP.
But once they have joined the ranks of IS, experts said, young recruits will find that trying to leave may cost them their lives.
More than 120 people were killed between October and December for trying to leave IS territory, according to the Syrian Observatory for Human Rights, a monitoring group.
"I conducted interviews with two young men who came back from (IS-held) Raqa and other areas, and it was clear they were resentful and felt like they had been tricked," Khatib said.
Instead of the Islamic utopia IS had promised, these young men encountered an area whose governance "was based on oppression and tells them not to think".
"They felt like they had been sold an empty project."