Since David Letterman's announcement last week that he will retire next year, many pop culture critics have taken the opportunity to toast the 67-year-old iconoclast and mainstay of late-night television. ​But amid the tributes, there has also been broad acknowledgement that the tone and format of late-night TV has changed dramatically since "Dave" made his TV debut nearly 35 years ago.

Nowadays, it's no longer enough for a night-time host to attract high-wattage stars and crack wise about the day's events for an hour to a live studio audience.

The success of more tech-savvy hosts such as Jimmy Fallon, Jimmy Kimmel and Stephen Colbert, who was recently named as Letterman's replacement, emphasizes that it's now equally important, if not more, to maintain a lively social media presence and produce a constant stream of online video.

"The world has absolutely shifted underneath [Letterman's] feet," says Sue Newhook, an assistant professor in the school of journalism at the University of King's College in Halifax.

​Letterman announced April 3 that he would leave his show, Late Show with David Letterman, around the conclusion of his current contract with CBS, which ends in August 2015. When he vacates his iconic desk on the stage of the Ed Sullivan Theater, it will mark the end of nearly 33 years as host of a nightly talk show.

Known for his gap-toothed grin and sarcastic asides, Letterman cultivated a role as the knowing cynic who didn't always indulge the narcissism and self-promotional impulses of his guests.

"He was like a smack upside the head for late-night television when he started out," says Newhook. "He made it lippier, saucier, and lots of people have been talking about his trademark irony. I think he smartened up late-night TV in a lot of ways."

A more 'multiplatform, multimedia format'

But in recent years, late-night talk shows have changed from a self-contained hour of celebrity interviews to brands that extend beyond the confines of the TV broadcast, says Emily Gagne, web editor for TV Guide Canada.

"I think that it's becoming a more multiplatform, multimedia format," says Gagne.

She says the focus has shifted to producing viral videos — mainly comedic sketches and musical numbers — that are watched and shared on the internet for weeks after appearing on TV.

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Late-night host Jimmy Kimmel has gained a reputation for pranking the media with fake video clips. (Dan Steinberg/Invision/Associated Press)

Gagne says one of the first talk-show clips to make a huge impression online was I'm F***ing Matt Damon, a satirical song that appeared on ABC's Jimmy Kimmel Live! in January 2008. Sung by Kimmel's then-girlfriend, comedian Sarah Silverman, this lewd musical number became a web sensation.

About three weeks later, Kimmel posted I'm F***ing Ben Affleck ­– a rebuttal, of sorts – which also went viral. The two videos have since garnered a combined 10 million page views on the Jimmy Kimmel Live YouTube channel, which doesn't include all of the other places online where it may have been posted and watched. (I'm F***ing Matt Damon won an Emmy award, for Outstanding Original Music and Lyrics, in 2008.)

Over at NBC, Jimmy Fallon has also taken full advantage of the online space. An alumnus of Saturday Night Live and now the host of The Tonight Show – once Johnny Carson's roost – Fallon has earned a reputation as a gifted musician and impressionist who regularly performs parody songs with guests as well as house band The Roots. Most, if not all, of these are shared online.

The show's most memorable clips include a number of medleys Fallon did with Justin Timberlake covering the history of hip-hop, and a recent segment in which actor Kevin Bacon recreated a dance sequence from his 1984 film Footloose, which has been viewed 12.5 million times since it was posted on March 21.

All about online engagement

This late-night TV focus on online engagement reflects a concerted effort to transcend the traditional talk-show format. Colbert, for example, has gotten a lot of mileage out of his gags online, including a series of fake ads in 2012 promoting - and attacking - the Colbert SuperPAC (political action committee). 

Last week, Alex Carloss, YouTube's global head of entertainment, told a television industry conference in Cannes that existing viewership for late-night TV shows has peaked, which is why hosts such as Colbert, Kimmel and Fallon, as well as Conan O'Brien (on the TBS network) and relative newcomer Seth Meyers (NBC), are looking online for ways to expand their audience.

Many of those who go to a show's YouTube channel are not the people who watch the nightly TV broadcast, says Paul Snow, manager of TV and film partnerships for YouTube. He says it's the difference between a show's "audience" and its "fanbase." 

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Jimmy Fallon has gotten a lot of mileage online with his musical parodies. (Diane Bondareff/Associated Press)

Someone who watches a segment on YouTube behaves "more like somebody who would belong to a fan club, as opposed to someone passively watching content," says Snow, whose team meets regularly with Kimmel's producers to discuss how to maximize online engagement. (Snow confirms that a separate YouTube team is working with Fallon.)

Snow says the four-million-plus people who subscribe to Kimmel's YouTube channel, for example, are more likely than the TV audience to share online clips and create their own show-inspired content.

Kimmel's producers have even looked beyond their own YouTube channel, producing a number of legitimate-seeming news clips that are picked up by traditional media only to be revealed by Kimmel as hoaxes. They include a viral video of a regrettable (and fake) twerking accident and one about a wolf prowling the Olympic Village during the recent Winter Games in Sochi, Russia.

Online one-upmanship

Gagne believes Letterman probably felt like he couldn't keep up in this era of online one-upmanship. Part of it is a question of personality and temperament: Letterman has always been more of a cool observer, whereas Kimmel and Colbert are pranksters and Fallon is a crowd-pleasing entertainer, qualities that translate well online.

Colbert is "definitely more experienced than Dave ever was in that realm," says Gagne.

"Letterman is a great interviewer and he does his top-10 [list] thing, and that's his niche. But now you need to do a little bit more and engage a little bit more."

The fact is, Letterman simply hasn't exploited the online space the way his competitors have, says Newhook.

"Letterman is clearly not the slightest bit interested in playing that kind of game, in leveraging social media to get the show out to the world."