The number of television viewers is plummeting, the Associated Press reports. About 2.5 million fewer people have been watching the main networks compared with last year.

NBC set a record last month for its least-watched week during the past 20 years, and maybe ever -- then broke it a week later. This is the least popular season ever for CBS' "Survivor." ABC's "Lost" has lost nearly half its live audience -- more than 10 million people -- from the days it was a sensation. "The Sopranos" (a show that has earned broadcast-network-like ratings in the past) is ending on HBO, and the response is a collective yawn.

The shift may cause traditional television advertisers to rethink their spending.

There could be many reasons for the drop in viewership, the article says. An earlier daylight savings time is partly responsible.

But the article highlighted some measurement problems. Nielsen doesn't count people who download a show on Apple's iTunes or stream it from a network's Web site.

And Nielsen is trying to include the estimated 17 percent of homes that have digital video recorders, but it only counts a show if the viewer watches it within 24 hours from when it aired. That's screwing up the numbers for heavily recorded shows like "The Office."