Infectious: The charm of Jimmy Kimmel

"He was the most powerful man in the world, but he had ONE weakness: an inability to speak. "So begins YouTube's viral video of the moment, a spoof trailer pitching a film called The President's Speech, a comic reworking of The King's Speech in which the central character, King George VI, has been replaced by George W Bush.

The two-minute clip, laced with ponderous voiceovers and archive footage of classic "Dubya" malapropisms, was created by Jimmy Kimmel, the late-night US chat-show host. For reasons that were never made entirely clear, it co-starred Mike Tyson, who took the Geoffrey Rush role. Thanks to the wonders of email, Twitter and other means of mass circulation, it has been chortled over by millions of jolly viewers.

And there's the rub. For when the executives who control the entertainment business take out their calculators and gaze at the profit and loss accounts, they'll realise that these several million online viewers represent rather more than the number of Americans who can be persuaded to tune their TVs to ABC at midnight, when Kimmel's urbane chat-show begins its hour-long slot.

In fact, The President's Speech is the latest in a series of headline-grabbing video clips which have highlighted a growing problem for the inhabitants of the nation's most famous sofas. Since all the best segments of late-night shows can be watched on the internet the following day, what's the point of bothering to sit through the programmes?

Ratings for chat-show hosts like David Letterman and Jay Leno have been on the wane for years, thanks to an increased use of digital recording machines and the endless fragmentation of TV audiences due to an explosion in the number of cable channels that offer an alternative to their network rivals. The exponential rise of internet streaming sites seems to be severely hastening that demise.

Today, Leno gets 4 million viewers (down from around 6 million five years ago). Letterman gets 3 million (down from 4.5 million). Kimmel, for all his fame and influence, pulls 1.7 million, which is roughly the same as his rivals Jimmy Fallon and Craig Ferguson. Conan O'Brien, whose new TBS show launched with huge fanfare in November, bagging 4 million viewers, is now stuttering along at around the 1.3 million mark.

The problem for chat-shows is they might have been purpose-built for YouTube. Short comic monologues, celebrity interviews, pre-recorded humorous sketches – are perfect for filleting into small segments. In 10 minutes at their laptop, fans can grabhighlights from last night's talkies minus the dull bits and adverts. If you want to know what bits of Stephen Colbert or Jon Stewart's Daily Show will provide tomorrow's water-cooler chat, simply watch the clips chosen for streaming on Gawker or The Huffington Post.

Unfortunately, online viewers don't contribute to official ratings. And they tend not to watch commercials. So even the most successful viral hits have only a minimal effect on the amount of cash a broadcaster makes. Given that "business," as the saying goes, is two thirds of the word "showbusiness," that's a problem.

The irony, for Kimmel, is that his career trajectory owes much to some high-profile YouTube hits. He took over ABC's late-night slot in 2003, at the age of 35, and initially suffered from lukewarm reviews. At the very start of the first episode, screened after that year's Superbowl, he even joked: "Welcome to 'Enjoy It While It Lasts,' my new talk show."

Over time, though, he found his feet along with a small, but loyal audience who admired his low key interviewing style and knack for persuading modish bands to perform on the show. When YouTube came along, it seemed a perfect medium for promoting the programme via clips of his bestcomic sketches.

Kimmel's first real breakout success in that medium came courtesy of his then-girlfriend Sarah Silverman, who in 2008 recorded a musical sketch with the actor Matt Damon, in which they claimed to be having sexual relations. The clip, entitled "I'm fucking Matt Damon," clocked up eight million "views" in a matter of weeks, and led to a short-term increase in viewers. A well-crafted reply, in which Kimmel persuaded Damon's friend Ben Affleck to record a spoof version of the song "We are the World," with the chorus "I'm fucking Ben Affleck" garnered another seven million views, and endless more helpful column inches.

More recent viral hits includea spoof of the Twilight films starring the cast of MTV's Jersey Shore, called "Frigging Twilight," and a studio sketch in which musician Joah Groban played piano, while singing twitter messages recently posted by the rapper Kanye West.

The most viewers of all – some 15 million – have enjoyed recent footage of Kimmel introducing the singer Justin Bieber to a three-year-old fan, who promptly asks for his hand in marriage. Lately, though, even Kimmel has started to publicly wonder if his online success could nowadays be diluting viewership of the show.

In January, at the Television Critics Association's annual press tour, he was asked ifhe was surprised by the amount of people who watchhis highlight clips online. His reply was brief, but to the point. He laughed, proclaimed himself astounded by the way millions of people find the clips, and then added whimsically: "It makes me think, why would they ever watch the show?"