For a while, it had the biggest buzz on the schoolyard. Was there a 13-year-old in America who could not identify the provenance of "Cheesy Poofs" or hadn't uttered the phrase "Oh, my god, they killed Kenny"?

"South Park," the aggressively rude, proudly crude animated cable TV series about a group of third-graders, was a certifiable pop phenomenon. It was hip enough to rate the covers of Spin and Rolling Stone, mainstream enough to rate Newsweek's and TV Guide's. It was so popular it once beat the ratings for its competition on the broadcast networks--a considerable feat, given that "South Park" could be seen on cable in less than half of all U.S. homes. There was the inevitable merchandising, with more than $400 million in sales of T-shirts, hats and talking alarm watches that squawk, "Hey, [expletive]! Time to wake up!"

Now, in the blink of the culture's eye, "South Park" has become something else: yesterday's news.

Listen to Kevin Conto, a 14-year-old high school freshman who fits squarely into the show's young-and-male audience profile: "It was popular when it was new. Now it just seems like the same old thing. No one cares about it anymore."

Or Bobby Gindes, an 18-year-old from Bethesda: "Last year, I watched it for two months. It really got repetitive. I haven't watched it in six months."

In its own cometlike way, "South Park" has become this month's Hootie and the Blowfish. It's like Jolt cola, swing music, Pogs, Matt Drudge, Tickle Me Elmo, Jenny McCarthy, cyber cafes--another in the endless procession of roaring hot fads that, by the way, just flamed out. Public adoration is, of course, fleeting (all stories of this kind are required by law to cite Andy Warhol's 15-minutes-of-fame dictum at least once). But in the '90s, super-stardom is an increasingly devalued currency. Even the formerly with-it phrase used to describe the newly unhip--"so five minutes ago"--is so . . . well, you know.

In fact, the trend in fads is to increasing brevity. The interesting thing about "South Park's" meltdown isn't that it happened but that it has happened so quickly. The series debuted on the Comedy Central cable network in August 1997. It reached its peak--both among viewers and as an object of media adoration--within six months. It's been fading since then; Cartman, Stan, Kyle, et al. have averaged about 2.4 million viewers per episode this year, about half the audience the show attracted a year ago, and almost two-thirds off its peak early last year. That may be the fastest arc from hot to not in TV history.

By definition, fads are short-lived. But the classic fad cycle--insider discovery followed by media ballyhoo followed by mainstream acceptance followed by jaded rejection--used to take a few years. Now it's down to a few months, says Marian Salzman, who tracks trends for Young & Rubicam, the giant ad agency.

"The reality is we're living in Internet years, where time is much more compressed," says Salzman. "What use to take 20 years gets packed into 12 months now. It's so hard now to sustain any unifying message in the culture because people are tuned in to so many different things at once. We're information-overloaded."

Adds Jane Rinzler Buckingham, president of Youth Intelligence, a market research company: "Fads come and go so quickly because marketers and the media are throwing more and more stuff at us. Young people are being over-marketed to, and they're sensitive about it. . . . It's fair to say that the faster something rises, the faster it's likely to fall."

"South Park" fits a trajectory common in the music business, where "hot" will never be a synonym for "enduring." Hootie and the Blowfish's first album, for example, sold nearly 10 million copies in 1994, making it the most successful debut in music history. But the group's second album, in 1996, sold only 2.3 million, and its third release, last year, sold less than half that. A similar fade distinguishes once-hot groups Counting Crows, Spin Doctors, Blues Traveler and Soul Asylum. Remember Ace of Base? Five years ago this week, the Swedish quartet was the most popular group in the world, with the No. 1 selling album here and abroad.

Market researchers chalk up the attenuated national attention span to two factors: a booming economy and a relentless media onslaught.

America is a great fad nation primarily because it can be; increasing national wealth has created a rising demand for more goods and services, perfect conditions for new consumption trends, says Irma Zandl, whose eponymously named market research company tracks young consumers. "Take a look at the typical supermarket--there are thousands more products there than there used to be 10 or 20 years ago," she says. "As a society, we are more entrepreneurial. Everyone is looking for the next hot property. The payoff when [something] hits is so enormous."

At the same time, the machinery driving it all is bigger and more refined than ever. During the 1990s, four huge companies--Disney, Viacom, Time Warner and Rupert Murdoch's News Corp.--have matured into mammoth entertainment-media conglomerates, controlling movie and TV studios, cable and broadcast networks, book and magazine publishers (Time Warner and Viacom jointly own Comedy Central, the cable channel that carries "South Park"). The amassing of corporate assets gives these companies unprecedented power to cross-promote and cross-merchandise their "products" through a variety of media outlets.

"The X-Files," for example, is corporate-engineered faddism in motion. Produced by News Corp.'s 20th Century Fox studio, it airs on News Corp.'s Fox broadcast network domestically, and on News Corp.-owned satellite systems in foreign markets. Reruns of the program are split between Fox-owned and -operated TV stations and Fox's FX cable network. News Corp.'s HarperCollins subsidiary has published 18 "X-Files" books. Last year's "X-Files" movie was created and distributed by the Fox studio. And for good measure, News Corp.'s TV Guide has featured the program on its cover several times.

The rise-and-fall cycle is certainly familiar to Trey Parker, 29, and partner Matt Stone, 28.

The former college buddies from Colorado were a couple of self-described "broke loser guys" when they moved to Los Angeles in 1995 to break into show business. They first tried peddling an animated comedy short about Frosty the Snowman fighting Jesus. With $1,200 in borrowed funds, they refined this story into "The Spirit of Christmas," in which Christ takes on Santa in a foulmouthed fight. The video became an underground favorite and soon led to "South Park" and its attendant mania.

Now Parker is bemused to discover that he's been declared washed up. "It's so funny," he says without evident bitterness. "In the past two weeks, people have been calling us up and asking, 'Do you have any comment on why you suck now?' . . . When 'South Park' came out, we were suddenly stars. The story was just as much us as it was the show. Now Time magazine is doing this huge hatchet job on us."

Parker figures it has something to do with the media's build-them-up/tear-them-down ethos: "All of a sudden, we're rich guys, and no one roots for the rich guy. Everyone hates Bill Gates because he's rich and powerful."

Parker says he and Stone got some wise counsel from another shooting star of the animation world, Mike Judge, the creator of "Beavis and Butt-head" and "King of the Hill."

"Mike laid it out for us," says Parker. "He told us how this was going to work: 'They'll love you and it's going to be great. And then all of a sudden you'll turn around one day and you'll be called sell-outs.' . . . He said, 'I hope you're ignoring all the good press because it'll be hard to believe when it turns.' "

In fairness, "South Park" may not be what it used to be.

Upon its debut, some TV critics and a few intellectuals praised it. British writer Andrew Sullivan called it "undeniably, one of the truest representations of America in popular culture today. . . . It deals with American reality without euphemism or politically correct anxiety. South Park is a place where belief in aliens is real, where it never stops snowing and where civil libertarians are trying to ban Christmas."

The program did offer some amusing shocks and outlandish situations. In addition to four obnoxious kids, its characters included a sex-obsessed school chef (voiced by "Shaft" musician Isaac Hayes), a gay dog (voiced by actor George Clooney), and one named Mr. Hanky, who easily qualifies as the most revolting protagonist ever on a TV series.

Parker himself acknowledges that the novelty of the show wore off as Comedy Central played the same episodes over and over, a result of production problems caused largely by Parker and Stone's insistence on writing and voicing every episode. That became harder as the pair worked on other projects, including a "South Park" movie, a largely unseen film called "BASEketball" and the script for a follow-up to the film "Dumb and Dumber."

Says Parker, "Our new shows became less of an event because [preceding episodes] were run into the ground."

Other observers say the show never really advanced creatively. " 'South Park' wasn't able to deliver on too much beyond shock value," Zandl notes. "And shock value gets tiresome very quickly."

The program ceased to be a topic of conversation several months ago among the hundreds of teens Zandl surveys each month; it hasn't picked up since. Young people, she says, "are pretty sophisticated and smart. [But 'South Park'] was never a smart show that works on several levels, as 'The Simpsons' does."

Parker and Comedy Central executives point out that "South Park" is still, after pro wrestling, among the most popular regularly scheduled entertainment programs on cable. It's enough of a franchise that Comedy Central has been using it as the lead-in for new shows. "It's our 'Seinfeld,' " says Tony Fox, the network's spokesman.

That's stretching it a bit. "South Park's" audience is falling at a time when Comedy Central is available in a greater number of cable households. In fact, the network used the big word-of-mouth about the series last year to persuade cable operators to carry the channel; it's now shown in 58 million homes nationwide, up 10 million from a year ago (it's still not available on the District's cable system). In other words, more people could watch "South Park," but fewer people actually do.

Comedy Central will attempt to reinvigorate its signature series by airing six new episodes starting this week. It's also planning a big promotional build-up for Paramount's release later this month of the movie, called "South Park: Bigger, Longer and Uncut."

Based on her soundings of young people, Zandl says Paramount shouldn't bother.

"It's going to bomb," she predicts flatly. "It's like bringing out a 'Beavis and Butt-head' movie now. It's over for 'South Park.' It's just over."

CAPTION: Comedy Central's onetime hit "South Park": Facing up to the harsh realities of a fickle public.

CAPTION: A 1997 episode of the animated comedy planted punch lines on Jay Leno's chin.

CAPTION: "South Park" creators Matt Stone, left, and Trey Parker: Nowhere to go but down?

CAPTION: In its all-too-brief heyday, "South Park" found itself on the cover of magazines like Rolling Stone and Spin.