TOKYO – The good news is that the user-base for mobile porn is expanding rapidly, and will continue to grow at ever-escalating rates for the foreseeable future; the bad news is that the explosion in porn downloads is overloading mobile networks.

Case in point, Japan, a leader in mobile network infrastructure and investment, which is itself at the breaking point, capacity-wise, due to the 91 million Japanese who now surf the Internet using mobile devices to download movies, music, games and, increasingly, porn.

According to Bloomberg, despite very healthy profits, mobile carriers DoCoMo and KDDI Corp. have been forced to impose limits on the heaviest users as the country’s $74 billion network feels the strain.

“When you have unlimited data, you’re going to have an issue with capacity – it’s an issue that’s been waiting to happen,” Windsor Holden, principal analyst at Juniper Research Ltd., told Bloomberg. “It wouldn’t surprise me that it happens in Japan first because they’ve had 3G for so much longer.”

While there has been a boon in downloads of more mainstream fare, telecommunications experts agree that adult fare is shaping up to be a major online driver of traffic, downloads, revenue and network capacity.

Tim Smith, CEO of 3G service company Sairis Group KK and a Japanese telecommunications industry vet, told Bloomberg the largest Japanese adult content Internet sites have as many as 1,000 new customers a day, each paying as much as 10,000 yen as a sign-up fee, and that while music downloads are the “official big earner” in Japanese mobile commerce, more money is made through porn, dating sites and even fortune-telling services.

With total annual sales in the pornography market estimated at 100 billion yen, Japan has more than 1,000 companies producing adult-content movies, generating about 17,000 titles last year, according to Smith.

While curtailment of badnwidth usage may be the short-term solution to Japan’s current capacity problem, the underlying issue of how other networks around the world  – including the United States’ – will support the inevitable explosion in mobile use is only underscored by the fact that porn remains, as it always hes been, a leading indicator of where the market is headed.

“Whenever there is a new distribution method for adult content, adult content will go that medium,” said Juniper’s Holden. “It’s gone that way since cavemen drew adult pictures in the cave.