"Ten months after Inauguration Day, the trend is holding: For late-night hosts, being sharply critical of President Trump is a winning strategy. And that is bad news for 'The Tonight Show.'
Stephen Colbert, who has made Mr. Trump a nightly target, assumed the top position in the ratings race in February and has only increased his lead since then. His program, “The Late Show” on CBS, has taken viewers away from Jimmy Fallon, the cheerful host of NBC’s storied franchise, who has lost 21 percent of his audience year over year since the fall season began on Sept. 25. At the same time, Jimmy Kimmel has made ratings gains in the 11:35 p.m. slot on ABC.
The recent declines leave the host of 'The Tonight Show' in danger of losing his position as the second-place late-night host. Mr. Kimmel’s show has averaged 2.4 million viewers a night, compared with 2.6 million for Mr. Fallon.
A year ago, a battle between Mr. Fallon and Mr. Kimmel was almost unimaginable, given the venerable NBC franchise’s lead of one million total viewers over ABC’s late-night offering.
During the current season, Mr. Kimmel — who, like Mr. Colbert, has made himself into a politically engaged monologuist — has seen his audience grow by 4 percent. Recently, he enjoyed a wave of publicity and warm reviews after he dissected the legislative attempts to repeal the Affordable Care Act.
The size of Mr. Colbert’s entire audience has spiked along with his gains among younger viewers. During the sweeps period, he averaged 3.7 million total viewers a night — a 23 percent increase from a year ago."
The New York Times
November 28, 2017
As of March, 2018, the hosts' average viewership per night was as follows:
Colbert: 3.10 million
Fallon: 2.49 million
Kimmel: 2.34 million
Source: TV By The Numbers