(FORTUNE Magazine) – Michael Lafavore, the editor-in-chief of TV Guide, has a vexing problem. He can't find enough TV stars to put on the cover of his magazine. So TV Guide has published covers featuring movies, NASCAR, and the NFL. "It's become very hard to find a prime-time television star," Lafavore laments.

It's about to get worse. NBC's Friends, NBC's Frasier, and HBO's Sex in the City will all end their runs soon. Everybody Loves Raymond is in the last year of its contract with CBS, though it could be renewed. Either way, after Jennifer Aniston, Kelsey Grammer, Sarah Jessica Parker, and Ray Romano leave the airwaves, prime time will be populated mostly by unfamiliar faces and unknown names.

This is a dramatic turnabout from the days when the broadcast networks regularly churned out hits and created true stars--performers who could not go anywhere without attracting a crowd. During the 1980s and early 1990s, for example, stars like Jerry Seinfeld, Tim Allen, Roseanne, Ted Danson, Michael J. Fox, Bruce Willis, George Clooney, and Tom Selleck burst into pop culture.

The absence of powerful stars isn't all bad news for the networks. The days of paying sitcom stars $1 million per episode are probably over. But new hits and stars have always brought excitement to the networks, and the buzz they generated kept viewers coming back.

But today's viewing audience is so fragmented that it's difficult to create an instant hit and harder still to turn an unknown into a star. Hit shows today achieve only about a 25% share of the viewing audience. Ten years ago, top-rated Home Improvement captured 33%, and a decade before that The Cosby Show reached 50%. "Lower numbers are more acceptable now," says Gail Berman, president of Fox Entertainment. "What constitutes a hit in this environment would have been laughable years ago." So instead of real stars, the networks have settled for niche stars--Jennifer Garner of ABC's Alias has her fans, but most people don't know who she is, while Ashton Kutcher is better known as Demi Moore's boyfriend than as a performer on Fox's That '70s Show.

Fragmentation is only part of the problem. Today's hits tend to be ensemble dramas and reality shows. Neither is star-driven. CBS's CSI, the top-rated show on television, is a police procedural whose lead actors, William Petersen and Marg Helgenberger, are hardly household names. Reality shows like Fox's American Idol, CBS's Survivor, and ABC's The Bachelor turn ordinary people into stars, but only for a moment or two.

This isn't to say that there are no stars left on television. They just aren't actors. Oprah Winfrey, Jay Leno, David Letterman, Bill O'Reilly, Larry King, and the broadcast network anchors are all bona fide stars. TV's newest star is talk-show host Dr. Phil. And Donald Trump's on the cover of TV Guide. No wonder the networks are in trouble. --Marc Gunther