Facebook, with more than 1 billion users worldwide, is increasingly being used by shady gun dealers who find it easy to hide in this sprawling and lightly policed virtual marketplace. And they are not just individuals skirting gun-control laws to sell a pistol or rifle. Facebook groups have been used to trade anti-aircraft weapons, grenades, heavy machine guns and guided missiles in battleground regions of the Middle East, according to the New York Times.

The gun bazaars have been operating in strongholds of the Islamic State in Iraq and the Levant, among other places, with sellers offering "a seemingly bottomless assortment of weapons provided to Iraq's government forces by the Pentagon during the long American occupation," according to the Times.

Weapons supplied more recently by the U.S. to Syrian rebels have also surfaced in the social-media marketplace, offering ordnance ranging from assault rifles to a sophisticated antitank missile system. This trading violates the ban Facebook put in place in January on the private sale of weapons. But the company is finding enforcement increasingly difficult as black marketers discover that Facebook and other social-media sites offer far easier opportunities than do physical gun stores for customer contacts, the display of wares and the concealment of transactions.

The deals have been tracked in Libya by the private consulting firm Armament Research Services, with the Times investigating Web bazaars in Syria, Iraq and Yemen. Facebook, which mainly relies on user alerts for enforcement, quickly shut down six suspicious groups investigated by the Times, but there's no guarantee that dealers won't resurface under different identities.

Domestic firearms traffic on Facebook is no less persistent despite the company's ban, according to a Forbes report last month that found more than 100 suspicious groups seemingly intent on gun deals. Some coyly portray themselves as gun enthusiasts merely interested in chatting, but their real intent can come across with lines like: "Show off your stuff and how much you think it's worth."

Facebook chose to ban private sales of guns and ammunition after protests from gun-safety groups. Numerous gun bazaars have been shut down, according to Forbes, but Facebook has no automated keyword system to flag violators. Company officials say their enforcement of the ban is an evolving policy. It had better be. Alerts from Facebook users cannot contain this dangerous traffic.

FROM AN EDITORIAL IN THE NEW YORK TIMES