Netflix and others in the mainstream video space owe a huge part of their success to word-of-mouth, with people recommending the services to friends and Facebook followers. You’re much less likely to do that with adult content, argued Wantedlist (site not safe for work) co-founder Danny Ting when I talked to him a few days ago. "It’s a very private matter for most people," he said. 

Wantedlist started in the late 1990s by taking lots of cues from Netflix. The service initially just rented DVDs to mail to its customers and then eventually expanded to online streaming as well. It now offers an online-only all-you-can-eat subscription that largely consists of catalog titles, and also still maintains its DVD business. Sounds like Netflix all over again — except that the rapid transition to online subscriptions never happened.