Recently, however, the al-Qaida inspired group Tehrik-e-Taliban Pakistan has made strides at recruiting European fighters for attacks against the West, according to Noman Benotman, a former jihadist fighter who now works for the London-based Quillium Foundation.

NPR: Russia's Chechnya Has Seen Decades Of War, Terror

It lately has taken to shutting down groups and individuals who, Hoffman says, send spam e-mails to strangers just to build up their networks or take some of LinkedIn's revenue by recruiting other members without its permission.

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