The pornography business as a whole is estimated to be about $5 billion, a sharp drop from a decade ago. Steven Hirsch, the co-chairman of Vivid Entertainment, a prominent pornographic movie studio, said that while there remained a market for prerecorded movies and clips — available for download and through cable subscription — interactive entertainment, including camming, accounted for half of the sales in the industry.

Internet traffic numbers bear out the popularity of camming. According to Alexa, a site owned by Amazon that measures Internet traffic, LiveJasmin, which is based in Luxembourg, ranks as the 80th most popular site in the United States and 103rd globally. Compete.com, a unit of Kantar Media, reports LiveJasmin has around 25 million unique visitors from the United States per month. Compete.Com puts MyFreeCams at four million unique visitors per month, while Alexa ranks MyFreeCams as the 341st most popular site in the United States.

That traffic still pales by comparison to the draw of pornographic sites that offer free prerecorded content. Pornhub ranks 56th in the United States, but its prerecorded clips are free. Among the ways it makes money is by converting visitors to customers of cam rooms.

“Live cam has become the most prominent part of the industry,” said Alec Helmy, the publisher of Xbiz, a sex-trade industry journal, eclipsing previous forms of pornography in popularity if not yet in total revenue. “Camming is driving the adult industry.”

The sites make money through sales of tips.