Late Night Hosts React to Jimmy Fallon's 'Tonight Show' Debut

David Letterman during Monday's episode of "The Late Show"

UPDATED: David Letterman, Jimmy Kimmel, Conan O'Brien and Seth Meyers had words of support and congratulations for Jay Leno and his replacement.

Jimmy Fallon took to the airwaves for the first time hosting The Tonight Show on Monday in his highly anticipated debut, and while Hollywood stars such as Robert DeNiro, Will Smith, U2, Lady Gaga, and former New York mayor Rudy Giuliani helped him get off to an explosive start, other late night hosts also took notice.

During Monday's monologue on the Late Show on CBS, David Letterman referred to the absence of his longtime late night rival, Jay Leno, from NBC, saying, "First thing this morning, I get a call from my mom. She says 'David, did something happen to Jay?' What am I gonna do?" he asked the audience.

PHOTOS: Jimmy Fallon: From Early 'SNL' to Movie Star to 'Late Night'

There has long been animosity between the pair of TV greats, after many assumed that Letterman would take the hot seat of The Tonight Show in 1992 when Johnny Carson retired but Leno got the spot instead.

However, Letterman expressed his respect for Leno earlier this month when he said, "He was a host on The Tonight Show for 22 years and now retiring," he said. "Congratulations on a wonderful run."

Fallon's former competitor, Craig Ferguson, who hosts CBS' Late Late Show, also made a couple of jokes in his monologue about the late-night shift at NBC.

After a few Presidents' Day remarks, in which he joked that all of the former presidents live together in a clubhouse, Ferguson said that all of the late-night hosts also live together in a clubhouse.

"I haven't seen Leno recently, though," Ferguson continued. "Although, one of his cars is still in the driveway, so maybe he's coming back."

Ferguson also alluded to NBC's previous decision to have Conan O'Brien succeed Leno as host of The Tonight Show, which ended up being short-lived, with O'Brien leaving to go to TBS and Leno returning to the Tonight Show job.

Referring to Bob Costas returning to host NBC's primetime Olympics coverage after a bout with an eye infection that forced Matt Lauer and Meredith Vieira to fill in, Ferguson said, "I'm surprised NBC let him back. Usually when they replace a host, they stick with their decision."

STORY: Inside Jimmy Fallon's Star-Filled 'Tonight Show' Debut

Jimmy Kimmel didn't mention Fallon's debut in his own opening monologue of Jimmy Kimmel Live! that began 30 minutes earlier, but he did tweet his support on Monday morning:

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While his own show was a rerun on Monday, O'Brien was quick to congratulate Fallon on Twitter:

Seth Meyers, who will debut as host of Late Night With Seth Meyers on Feb. 24, congratulated his former Saturday Night Live costar by reminding his Twitter followers to tune in for Fallon's first night on the job: