When a lone terrorist slaughtered 38 tourists at a Tunisian resort on June 26, the Islamic State turned to one of America's leading social-media companies to claim responsibility and warn of more attacks on the world's nonbelievers.

"It was a painful strike and a message stained with blood," the Islamic State announced on Twitter following the massacre in Sousse, a popular destination for Europeans on the Mediterranean. "Let them wait for the glad tidings of what will harm them in the coming days, Allah permitting."

Three days before the assault, the Islamic State relied on another popular U.S. social-media platform, Google's YouTube, to promote a grisly propaganda video of three separate mass killings. Men accused of cooperating with U.S.-coordinated airstrikes in Iraq and Syria are seen being incinerated in a car, drowned in a cage lowered into a swimming pool and decapitated by explosive necklaces looped around their necks.

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