Rupert Murdoch’s Fox TV network has a problem or three: falling revenues and no profits would be at the top of the worry list, but now there’s an even greater concern: its biggest franchise, American Idol, has just ended and the two final nights were among the least watched since the program started as a summer program around eight years ago.

Kris Allen’s win in Wednesday night’s season finale attracted 28.8 million viewers. The number was down from the 31.7 million that tuned in to last year’s finale. Fox still won and Idol was the dominant series of the US season which finishes tomorrow night (So You Think You Can Dance starts tonight in the US on Fox, kicking off the summer season of US TV).

US reports say the viewer total was the second lowest for an Idol finale since its first season, while Tuesday night’s final performance show was also the least-watched since season one.

More importantly, it was the lowest rating ever final in the most important demographic in US TV, the 18-49 age group. But still beat the competition by a wide margin. The program ran seven minutes over past the slated 10 pm (Eastern) finish as Fox attempted to give its ratings a boost. Nielsen figures showed the program peaked at 34 million viewers in the last half hour when the winner was announced.

It’s a bad sign of Fox, even though its ensured that the network topped the 18 to 49 age group again, its margin was down.

No reality programs, from drama oriented Big Brother to performance oriented Idol, have the longevity a soap or drama can have. They are very profitable, but run out of steam as viewers become indifferent. The next Idol will be the big test of Fox’s ability to keep the format fresh and maintain viewing levels