Under pressure from gun control advocates, Facebook agreed Wednesday to delete posts from users selling illegal guns or offering weapons for sale without background checks.
A similar policy will be applied to Instagram, the company's photo-sharing network, Facebook said. The measures will be put into effect over the next few weeks and will apply worldwide at Facebook, which claims 1.3 billion active users.
"We will remove reported posts that explicitly indicate a specific attempt to evade or help others evade the law,'' the company said in a statement.
New York Attorney General Eric Schneiderman has been asking Facebook to adopt such restrictions, as have Mayors Against Illegal Guns, the group backed by former New York Mayor Michael Bloomberg, and Moms Demand Action. Moms Demand Action collected more than 230,000 signatures on petitions calling on Facebook to act.
"Responsible social media sites know that it is in no one's interest for their sites to become the 21st-century black market in dangerous and illegal goods that place our families and communities at risk,'' Schneiderman said.
"I congratulate Facebook and Instagram for taking these simple, common-sense steps to protect the safety and security of their users, and encourage other social media sites to follow their lead.''
Tom King, president of the New York affiliate of the National Rifle Association, acknowledged that Facebook is allowed to set its own rules but said he regards the new restrictions as "a kind of limit on our First Amendment rights.''
Although his group, the New York State Rifle & Pistol Association, doesn't sell guns, he wondered whether it could be blocked if somebody reported the organization's Facebook page.
"This is something that could greatly get out of control very quickly,'' King said.
New York has some of the nation's toughest gun laws. It requires a background check for private gun sales and prohibits sales of some popular firearms, such as the AR-15 rifle.
Under the new policy, Facebook would allow a user in Texas to list an AR-15 for sale, since the gun is legal there, as long as it wasn't offered for sale in states where the weapon is illegal. But the company would delete a similar post from someone in New York.
It would also remove posts from any state in which a gun seller says a background check will be skipped, even if such checks aren't required where the seller lives.
"This is one of many areas where we face a difficult challenge balancing individuals' desire to express themselves on our services and recognizing that this speech may have consequences elsewhere,'' Facebook said.
The company already has systems in place to remove advertising that is false and deceptive, and it prohibits ads for illegal drugs, tobacco products and prescription drugs.
There's no way to know how many guns are sold via Facebook, because the transactions are actually completed offline, said John Feinblatt, chairman of Mayors Against Illegal Guns. But such sales have occurred.
In Kentucky, for example, federal authorities in February charged an Ohio man with illegally selling a 9 mm pistol to a Kentucky teenager in a transaction arranged through Facebook.
In an online petition, Moms Demand Action warned that Facebook and Instagram could also be used to sell guns to convicted felons, domestic abusers or others barred under federal law from obtaining a firearm. That list also includes people judged mentally defective.
Feinblatt said Google Plus and Craigslist already prohibit all gun sales, legal or illegal. But he said there are "virtual gun shows'' online. His group issued a report in December showing 66,000 active ads on a popular gun sales website called Armslist.
The report said 16 states and the District of Columbia require background checks for private firearm purchases.