The main obstacle is finding these Snapchat strippers. Most operate undercover, partly because pornography violates the company’s community guidelines. Last week, Snapchat published a “Snapchat Safety Center” reminding children that nude pictures were not allowed. “Don’t use Snapchat for any illegal shenanigans and if you’re under 18 or are Snapping with someone who might be: keep your clothes on!” the company wrote.

Snapchat seems to be enforcing its own rules. A few weeks ago, as a test, I added 30 Snapchat accounts that promised to share porn pictures, some at no charge, others for a fee. A week later, 28 of those accounts were shut down. “We have sophisticated systems in place to detect abuse and shut down accounts that violate our terms of use,” Snapchat said in a statement. “We are very aggressive about taking action and we will continue our efforts on that front.”

To avoid being removed from Snapchat, users have turned dozens of online sex forums into virtual swap meets. Strippers advertise their services there, and interested viewers post their Snapchat user names. This way the unclothed can vet the clothed before sending them pictures or receiving money in their Snapchat tip jars.

Some strippers are also avoiding expulsion by not charging users on Snapchat. Instead, they offer a daily tease on the messaging app, then lure people to their personal websites, where they accept payment from third-party services like PayPal and Amazon.

Snapchat isn’t the only mobile app on which pornography has covertly flourished. The messaging app Kik, which allows people to chat using text and photos and to share links to webcam chats, has also become a playground for porn. And while Twitter and Facebook don’t allow people to send money, both are popular with porn actors who want to engage with fans.

And of course, pornography is not new to the Internet. It seems as if webcams were invented solely to allow people to pay to see someone else naked. But what’s changing now is the rapid shift to mobile. According to a recent study by Juniper Research, video chats and subscription services on mobile devices will account for $2.8 billion in porn-related revenue this year.