(Gabrielle Giffords and Mark Kelly on “20/20” (ABC))

If you land what is maybe the biggest news “get” in ages, but it airs on NBC in primetime, did it actually happen?

The magnitude of the damage that has been done to the once dominant but now mired-in-fourth-place NBC over the past several years was made painfully clear Monday night, when its stunning coup — the first interview with former Penn State coach Jerry Sandusky since the sex abuse scandal accusations — copped a paltry 3.9 million people.

At the same time, 12.4 million were watching ABC’s canned “20/20” special detailing Rep. Gabrielle Giffords’s (D-Ariz.) recovery since being shot in the head in January.

That’s whopping 218 percent bigger crowd than tuned in to NBC’s “Rock Center” interview with Sandusky, in which he admitted to “showering and horsing around with young boys,” but insisted he’s innocent of the 40 counts of sexually abusing eight boys.

NBC had had only a few hours in which to promote its late-breaking interview on an unfolding and highly incendiary story.

Of course a network’s most efficient means of quickly promoting some huge “get” is on its own air. But, sadly for NBC, between 8 and 10 p.m. Monday a paltry 4.2 million people were watching the network’s former holiday program turned fourth-quarter player, “The Sing Off.”

(And, if NBC News was smart, it also promoted the upcoming interview on its cable news channel MSNBC — though that would only have gotten word of the incredible upcoming interview out to another 900,000 viewers or so.)

Meanwhile, in those same two hours, ABC was promoting the heck out of its Giffords piece to a massive “Dancing with the Stars’” audience of 18 million.

In the interview, with NBC’s Bob Costas, Sandusky described the night graduate assistant Mike McQueary said he walked in on Sandusky allegedly raping a 10-year-old in the locker room shower:

“We were showering, and horsing around, and [the child] actually turned all the showers on, and was, uh, actually sliding across the floor. And we were—as I recall—possibly snapping a towel. Horseplay.”

Joe Paterno’s one-time defensive coordinator, and founder of a charity to help at-risk youth, has been charged with 40 counts of sexually abusing eight boys dating back as far as 1994, A grand jury report detailed claims of alleged sexual encounters with young boys in Sandusky’s house, in hotels, and in Penn State locker rooms.

But at 11 p.m., the table turned on the two stories – at least, in Washington.

More people in Washington learned details of Bob Costas’ phoner with Sandusky by watching the lead story of NBC-owned WRC’s late local news than had actually watched the interview immediately before the station’s late local newscast. In Washington, 100,000 people watched the Costas interview on “Rock Center” – 131,000 watched WRC’s newscast lead story about the interview.

Meanwhile, ABC affiliate WJLA opened its late local news with a package heaving on Giffords material immediately after the Giffords special – including reactions from various members of Congress to her recovery. But while nearly 190,000 people had caught the Diane Sawyer hour on Giffords, only 99,000 watched the local-newscast that followed.

Monday evening’s ratings story in Washington played out the same among the 25-54 year old viewers who are the target audience of news programming.