Victor Keegan writes in The Guardian:
Tomi Ahonen, a strategy consultant, points out that whereas porn and gambling drove revenues on the internet, five content groups are more successful than adult material on mobile phones: music, infotainment, images, videogames and web browsing. He reminds us that in 2005 one annoying ringtone, Crazy Frog, outsold all of iTunes. A key reason is that most content on the web is free whereas mobile phones arrived with a payment system pre-installed for calls, followed by a premium service for texting. If the web had had its own payment system it would have taken a different course.