Cable news used to be a growth sector for the media industry. Now it's shrinking.

According to Pew's annual study on the news media, all three major cable news channels--CNN, MSNBC, and Fox News--lost viewers in 2010 for the first time in at least a dozen years, in the biggest year-to-year decline in viewership since Pew began collecting data on cable news in 1997.

CNN fared worst in 2010, losing 37 percent of its primetime viewers, while Fox News shed 11 percent and MSNBC only 5 percent, allowing it to overtake CNN for second place behind the still-dominant Fox. The data indicates that Fox has experienced only two ratings drops since 2001, and that both have come during midterm election years. MSNBC viewership, meanwhile, soared in the lead-up to the 2008 presidential election and has since stayed relatively flat, while CNN's audience has plummeted following the presidential election.

What explains cable news' dismal showing? Analysts have offered four main explanations:

Two-Trick Pony  Outside the Beltway's Doug Mataconis says that nowadays, cable news has mainly become a source for (sometimes irrelevant) breaking news and "propaganda for the left or the right."  The number of people that "watch any of these cable news networks is a small part of the television audience as a whole," he adds, "and it isn’t hard to see why."

Limit to Partisan News  The Hollywood Reporter explains cable news was supposed to differentiate itself from instant news on the web and mobile devices by oferring "primetime with a point of view"--a strategy CNN spurned in a widely criticized move. But Pew's Amy S. Mitchell asserts that "there just may be some limitation to the population that's going to come to those personality driven shows day in and day out."

Diminishing Destination for Breaking News  Pew notes that cable news' average viewership--a metric often skewed by sudden audience spikes when big news breaks--has declined, suggesting that fewer people are turning to cable when breaking news events occur.

The Growth of the Internet  If people are turning to cable news less for breaking news, there's a good chance they're going online instead. The web was the only medium measured by Pew to gain in audience in 2010, growing by 17 percent. A survey found that 34 percent of Americans got their news online "yesterday," while 58 percent said they had tuned in to broadcast or cable news the day before the survey. "The internet now trails only television among American adults as a destination for news," Pew observes, "and the trend line shows the gap closing." The trend will only accelerate with the proliferation of digital devices. As MinnPost points out, 47 percent of Americans now get news from a mobile device.

Not all is lost for cable news channels, however. While their audiences declined, Pew found that revenues and profits were actually up, in large part due to the fees the networks receive from cable providers. In another notable finding, Fox, for the first time, spent more moeny on news than CNN, though CNN invested more in foreign and domestic bureaus.