Six months ago, a 15-year-old boy in Kentucky logged into Facebook and arranged to buy a gun. He found a seller – one of many – willing to complete the sale without a background check. The seller was even willing to cross state lines from Ohio to make the sale work. Just a week later, the gun found its way to a high-school football game. In the hands of a student.

I’m a parent, so of course that scares me. And not just as the mother who runs Moms Demand Action for Gun Sense in America, who wonders how to help stop the next Sandy Hook tragedy. I’m a parent who, like most others, knows that even plugged-in teenagers can tire of Facebook for the next best thing online.

So of course I was gratified that Facebook and its photo-sharing subsidiary Instagram responded to our campaign and agreed last week to prevent potentially deadly gun sales with no background checks on their platforms. Because there’s a big problem when children can buy guns on the same sites where they “like” photos and chat with friends.

But Facebook and Instagram aren’t the only places on the internet where minors and criminals turn to find guns for sale without background checks. In fact, research by our parent organization at Mayors Against Illegal Guns has shown that plenty of online gun buyers – 1 out of 30 on Armslist.com – possess criminal records that federally prohibit them from owning firearms.

And while Facebook will now block gun-sale postings to users under 18, and all posts that reference “no background checks” will be deleted immediately, it’s still almost impossible to measure just how easy it is for minors, felons and other dangerous people to get guns online thanks to the so-called “private-sale loophole”.

If you’re not familiar with the loophole, it’s the dangerous gap in our gun laws that exempts unlicensed gun sellers online and at gun shows from conducting the background checks that are required at federally licensed dealers. Sixteen states have already expanded background checks to private sales, and a majority of senators vowed to close this loophole last April – with the support of 90 percent of Americans – but the gun lobby convinced enough senators to block the bill from reaching a full vote.

It’s hard to overstate the importance of Facebook’s decision. The company recognizes what Congress – and the 34 states that have not yet closed the private sale loophole on their own – have not: background checks work. Consider Missouri, where new research shows that the state’s firearm murder rate rose 23% in the years immediately following the 2007 repeal of the state’s background check law. Selling guns with no questions asked is too dangerous for our kids, it’s too dangerous for our communities, and now it’s too dangerous for the world’s largest social network.

But kids can still find guns on Armslist and many other gun sites. And while respecting free-speech issues, we’re happy to listen to more companies where our children are spending increasing amounts of time.

It was less than six months ago that we completed our campaign to get Starbucks to change its policy of allowing customers to openly carry gun in its stores. We’re going to keep applying pressure to corporations – as we have with Starbucks and Facebook – as long as episodes of gun violence continue to kill 33 Americans a day. On our side we’ve got the momentum, the public support and the evidence that these measures work. Now it’s time to close loopholes on every device, app and storefront where a kid with a login can become a kid with a gun.