How bad are things for the broadcast networks this summer? So bad, apparently, that entire groups of viewers have decided to abandon the networks completely.

In what will likely be the low-water mark for the Big Four this summer, CBS and NBC both generated demographic hash marks during the 8 p.m. hour on Saturday. That means the networks averaged a 0 rating and a 0 share in at least one demo group, according to preliminary Nielsen fast national data.

Not surprisingly, the sliver of the viewing audience that literally didn’t register with Nielsen on the July 4th holiday: Teenagers, aged 12-17.

For NBC, the show that snagged a goose egg was the long-ago cancelled drama "Kings." During the 8-8:30 p.m. half hour, it earned a 0.0/0 among men 12-17 and all adults 12-17. (Teen girls were a bit more enthusiastic, giving the show a 0.1/1).

Over at CBS, "48 Hours Mystery" was the program cursed with no teens from 8-8:30 p.m. It also notched (or is that failed to notch?) a big 0.0/0 with all viewers 12-17 and women 12-17. (Teen boys handed "48" a 0.1/1).

Both shows also scored goose eggs for the full 8 p.m. hour, with "Kings" zeroing out among men 12-17 and "48" doing a swan dive with women 12-17.

There are plenty of caveats that go with these numbers, of course.

It was a national holiday in the middle of the summer, and it fell on the weakest night for networks (Saturdays). The only demographic to register a zero rating was teenagers, a group not particularly coveted by most advertisers. And dozens of cable networks no doubt flatlined as well.

Still, the Saturday numbers do underscore just how rough things have been for broadcasters this summer. Adults 18-49 ratings below a 0.5 have become common for many shows in many time slots.

Indeed, for anyone who thinks it’s stretching things a bit to draw conclusions from holiday ratings among teenagers, consider this: NBC’s "Kings" drew only a 0.2/1 among the key demographic of adults 18-49. That’s not hash marks, but it’s pretty darn close.

What’s more troubling for the networks: The ratings misery comes during a summer in which many networks are offering up a healthy dose of first-run scripted programming. Some of it is burn-off theater, but some shows have been given big promotional pushes ("Mental," "The Philanthropist").

Viewers, however, just aren’t into what the broadcasters are serving up this summer.

You can bet planning meetings are already underway at several networks to figure out just what can be done to avoid this mess next summer.

Meanwhile, here’s something up beat and peppy for those network types depressed by the numbers: