Her broadcast averaged 7.04 million viewers last week, third to NBC's "Nightly News" (8.56 million) and ABC's "World News" (7.97 million), according to Nielsen Media Research.
CBS points out that the "CBS Evening News" is the only one of the three network newscasts with more viewers last week than the same week a year earlier. NBC's margin of victory last year was 2.2 million.
"Where we were last week or even in the weeks to come is not as important as where we are next year and even the year after that," said CBS News President Sean McManus. "As we have said from the very beginning, we are focused on the long term and on making slow and steady progress, which we are doing."
But the steady erosion is a discouraging sign, especially after 13.6 million people tuned in for Couric's first broadcast.
Last week was a strong one for ABC's "World News," which sent anchor Charles Gibson to Amish country to follow up the school shooting story; neither Couric nor Brian Williams traveled for that story. ABC News may also have benefited from Brian Ross leading the way on the scandal involving congressional pages.
The week before, the only other week Gibson had defeated Couric in the ratings, it was by only 100,000 viewers.
On Tuesday, when Gibson was in Pennsylvania, "World News" tied NBC's "Nightly News" in the ratings, well ahead of CBS.
Some in the news industry were surprised that CBS didn't send Couric to that story, particularly given her work on the scene of the Columbine school shooting for the "Today" show was among her most memorable.
Executives considered sending Couric to Pennsylvania, said Rome Hartman, executive producer of the "CBS Evening News," but he said that with the congressional page scandal, the Amish tragedy wasn't the only major story of the week. He said he believed CBS reported more extensively on the shooting the day it happened than any of its rivals.
Asked whether he believed the decision to keep Couric in New York hurt CBS, Hartman said: "You can overthink things, and I'm trying not to, because it's such a long-haul deal."