A week after Couric’s “CBS Evening News” dipped to its worst viewership ever, NBC’s “Nightly News with Brian Williams” matched its worst viewership among adults 25-54 in at least the past 20 years, since Nielsen began using people meters in 1987.

“Nightly” averaged 2.18 million 25-54s last week, the week ended May 13. The newscast also saw its total viewers average slide to its lowest point in months, 7.16 million.

That’s a 15 percent decline compared with the same week a year ago. By comparison, ABC’s “World News with Charles Gibson,” which finished first with an average 7.91 million viewers, was up 8 percent over the same time last year.

There’s likely no one reason for the slide, but it’s notable because NBC had been on top for years, up until Gibson began moving ahead three months ago.

Certainly part of NBC's slide could be the overall dip in TV viewership that's spreading across primetime and other dayparts. That's seen a number of top-rated programs, including NBC’s “Today,” decline.

But it could also be signaling a longer-term shift in viewing from the networks' nightly news shows.

The backbone of that audience has long been older viewers, those over 50. They've been the least inclined to follow younger people to cable and the internet and other new media options. But they might now be doing so.

Year to date, NBC’s newscast is down 6 percent in total viewers and CBS is down 5 percent. ABC is up 3 percent.

Yet even ABC is struggling with the viewers who matter most, those 25-54s, and last week Gibson too was near a record low.

One final reason for NBC's dip could be a lingering backlash over the network's use of footage of Virginia Tech shooter Seung Hui Cho’s final diatribe last month. The decision to air the footage was met with some public outrage, followed by weeks of debate over whether the network made the right move. Journalists agreed that it had, but the general public was more divided.

Though such things are rarely reflected so quickly in the ratings, NBC’s telecast has dipped a lot in the past few weeks; it usually takes the nightly news shows months to see any sort of up or down trend.

In the past two weeks, “Nightly” has gone from 7.89 million total viewers the week ended April 29 to 7.16 million last week, a difference of 730,000. ABC and CBS were both down fewer than 300,000 in that span.

Meanwhile, in other dayparts for the week ended May 6, NBC’s “Meet the Press” was yet again first among Sunday morning shows in terms of total viewers, bringing in 3.44 million, and among adults 25-54 with a 0.9 rating. ABC’s “This Week with George Stephanopoulos” was second in viewers with 2.42 million and tied for second among 25-54s with a 0.7 rating, followed by CBS’s “Face the Nation” with 2.38 million and a 0.7, and Fox “News Sunday” in fourth place with 1.13 million and a 0.3 among adults 25-54.
 
In late night, NBC’s “Tonight Show with Jay Leno” was first for the week, averaging 5.6 million total viewers and a 1.7 rating among adults 18-49. “The Late Show with David Letterman” on CBS had 4.0 million viewers and a 1.3 rating in the demo, with ABC’s “Nightline” bringing in 3.8 million viewers and a 1.2 rating. In late late night, NBC’s “Late Night with Conan O’Brien” had 2.4 million total viewers and a 1.0 in 18-49s, ABC’s “Jimmy Kimmel Live” had 2.0 million viewers and a 0.7, with CBS’s “The Late Late Show with Craig Ferguson” averaging 1.8 million viewers and a 0.6 and NBC’s “Last Call with Carson Daly” bringing in 1.4 million viewers and a 0.6 among 18-49s.
 
In morning shows, NBC’s “Today” was first with 5.6 million total viewers and a 4.4 household rating and 16 share, followed by ABC’s “Good Morning America” with 4.9 million viewers and a 3.8/13. CBS’s “Early Show” was third with 2.5 million total viewers and a 2.0/7.
 
In daytime, CBS had the largest audience for both daytime dramas and full daytime, 3.62 million and 3.97 million, respectively, but was second among women 18-49 for daytime dramas with a 1.4 rating and full daytime with a 1.3. ABC had the second-largest audience with 3.22 million watching its dramas and 3.34 million for full daytime, and was first among women 18-49 with a 1.6 rating for both dramas and full daytime. NBC had an audience of 2.06 million for both dramas and full daytime, and was third with a 1.1 rating in women 18-49 for both.