Porn

Youporn makes money via affiliate programs with VideoBox, AdultFriendFinder, a live cams producer and then other general traffic programs with various adult companies. I have no idea if they're profitable or not, though.

I suspect that you'll see a gradual drop-off in the number of adult tube sites because there are only so many affiliate programs around and if the conversion rates continue to drop, especially for mainstrain porn they just won't have enough money to pay their server and bandwidth costs and still make enough money to survive.

Some of them have hybrid business models, some just host blatant pirated content, some are trying to do things right...it's a real mix out there.