It is still too early to say Americans are tossing away their TV sets.

But clearly a growing number of people are giving up their sets, enough that Nielsen has recorded a decline in the number of households with TV sets for just the second time in history, the first being two decades ago and tied to a change in data collecting.

In its annual television audience report released this week, Nielsen noted that the number of TV households will drop next year, from 115.9 million in 2011 to 114.7 million next year.

That's a dip of only 1 percent, but it could be a harbinger of declines to come.

Over recent years the growth of TV households slowed notably, as more and more alternatives to TV viewing became available, such as online, mobile and tablets.

People are watching as much or more TV than ever; they're just watching more of it through other channels.

Those channels have always been seen as complementary to traditional TV viewing. This week's data suggests for the first time that those alternative delivery methods may actually be cannibalizing traditional TV viewing.

It's clearly a trend that's more prevalent among the young.

While overall households with TV sets dropped 1 percent, they fell 2.7 percent among adults 18-49.  

That younger subset is much more likely to watch online video, as studies have shown. GigaOM Pro says that roughly a third of all online video viewers are ages 18-32, while 30 percent are 33-44.

And a recent report from YPulse, a youth-targeted research firm, found that less than half of college students watch TV on actual TV sets.

These younger viewers, many of whom grew up with DVRs, are used to watching television on their own schedule. They consider TV a fluid medium that can be viewed across a range of platforms, rather than just in one place at one time.

It's likely that the decline in TV households will continue.

Nielsen does note that this has happened once before, in 1992, though that dip was tied to adjustments in data for the 1990 Census.

But the ditching of TV sets has a long way to go before it becomes commonplace.

Among older people, the traditional set is likely to remain the viewing media of choice, with other media options used only when a set is not available. Similarly, the traditional set will remain the device of choice among households with children, where group is commonplace.