Facebook (Nasdaq: FB) the world’s biggest online social network, and other social media are increasingly being used by terrorists to recruit sympathizers, spread propaganda and plan potential attacks, according to the United Nations.

“Promotion of extremist rhetoric encouraging violent acts is also a common trend across the growing range of Internet-based platforms that host user-generated content,” the UN Office on Drugs and Crime said Monday in a 148-page report, “The Use of the Internet for Terrorist Purposes.”

Facebook, Twitter Inc. and Google Inc.’s Youtube were also identified as primary conduits.

Counterterrorism officials met in Vienna for the release of the findings. The UN wants countries to ratify legislation to prevent so-called cyberterrorism and boost cross-border collaboration between law-enforcement agencies.

“Social networks like Facebook are taking on more and more relevance,” Germany’s top anti-terror official, Hans-Georg Maassen, said in prepared remarks. “Terrorists can develop ties irregardless of borders.”

Spokespeople at Facebook, Google and Twitter didn’t immediately return a phone calls and e-mails seeking comment.

U.S. senators had asked Facebook, Google and Apple Inc. to support a proposed compromise cybersecurity bill in July. Congress failed to pass the so-called Cybersecurity Act this year.

U.S. Defense Secretary Leon Panetta said Oct. 12 that the Pentagon and American intelligence agencies are seeing an increase in cyber threats that could become as devastating as the Sept. 11, 2001, attacks if they aren’t stopped.