Rachel Maddow on "The Tonight Show." Theo Wargo/Getty Images Fox News last week fell into third place during primetime in the coveted advertising demographic of 25-54 year-old viewers, the first time it had done so in 17 years, as rival network MSNBC surged into first place.

Adweek reported that last week was the first time that Fox finished third among cable-news networks in the coveted demo since June 2000, when the right-leaning network was still in its infancy.

According to Nielsen, MSNBC led the three cable networks between 8-11 p.m., averaging 611,000 viewers between the ages of 25 and 54, while dominating with 2,440,000 viewers overall.

CNN came in second with 589,000 in the coveted demographic, while remaining in third in total viewers with 1,649,000. Fox slid to to third with 497,000 viewers in the demo, and 2,405,000 overall.

Despite the primetime dip last week, Fox is still above MSNBC and CNN in primetime and total-day ratings this month.

The ratings slump came as primetime programs notably avoided and counter-programmed major news revelations last week, including reports that President Donald Trump disclosed classified information with Russian officials and that he had nudged former FBI director James Comey to lay off investigating former national security adviser Michael Flynn.

Last Monday, primetime Fox News hosts like Tucker Carlson and Sean Hannity largely ignored a report on Trump's revelation to Russian officials. They instead focused the bulk of their programs criticizing the media's coverage of the presidency, as well as instances of violence at protests and Comey's firing.

The same was largely the case on Tuesday, when both hosts again took aim at the media while MSNBC and CNN offered wall-to-wall coverage of a New York Times report regarding Trump's nudge for Comey to end the Flynn probe.

The news also came amid a larger shakeup of Fox's primetime schedule, which saw its highest ratings ever last quarter.

Last month, Carlson moved into the 8 p.m. time slot formerly occupied by Bill O'Reilly, who left the network amid sexual harassment allegations, while "The Five" moved into Carlson's 9 p.m. time slot.