The Big 3 networks' evening newscasts probably turned in their worst ratings week since TVs became a household staple during the week of May 18.

Last week (at NewsBusters; at BizzyBlog), based on the ratings for the week of May 11, I noted that the shows' combined audience had plunged a stunning 25% from their late January post-inauguration peak to a level barely above the 20 million mark.

At the time, I thought that we wouldn't see a sub-20 million week until sometine during the summer.

Well, it's not summer yet, and they're below a combined 20 million already. What's more, the results are even worse in the coveted 25-54 demographic.

Here's a comparison of the week of May 18 to both the previous week and the same week a year ago (Source: Media Bistro - May 18, 2009; May 11, 2009; May 19, 2008):

The excuse that last week was down only because of the upcoming Memorial Day weekend doesn't fly. Last year's drop from May 12 to May 19 (not charted above) was only 480,000 (230,000 in the 25-54 demo). This year's May 11 to May 18 drop of 830,000 (460,000 for 25-54) was much higher off of smaller base.

The trend within a trend, at least year over year, is that NBC's Nightly News with Brian Williams and ABC's World News Tonight with Charles Gibson have been losing viewers, while Katie Couric's CBS Evening News appears to have bottomed out. Brian and Charlie are coming down to Katie's level at a surprisingly fast clip. NBC has lost 30% of its lead over CBS; ABC's lead has dropped by over 40%.

Media Bistro, the source of the data, seems oddly ignorant of the precipitous downward slide, nonchalantly telling its readers only that "All three networks were declined week-to-week in both categories," and that "Year-to-year, the only program to gain viewers is the CBS Evening News with Katie Couric." That's light treatment indeed of year-over-year 9.4% and 12.4% drops at NBC and ABC, respectively, as well as 18%-plus drops at the two networks in the 25-54 demo.

Networks execs nevertheless surely know that their former crown jewels are fading, and quickly. Perhaps it will occur to one or more of them that viewers are tiring of their years of insufferable bias, or of their more recent almost all-Obama, all-the-time obsession. Nah -- If they haven't figured it out by now, they likely never will.

Cross-posted at BizzyBlog.com.

—Tom Blumer is president of a training and development company in Mason, Ohio, and is a contributing editor to NewsBusters