By 2010, traditional TV advertising will be one-third as effective as it was in 1990, according to a study from McKinsey & Co.

That forecast assumes a 15 percent decrease in buying power driven by CPM rate increases; a 23 percent decline in ads viewed due to switching off; a nine percent loss of attention to ads due to increased multitasking; and a 37 percent decrease in message impact due to saturation, AdAge reports (via MediaBuyerPlanner). According to McKinsey, real ad spending on prime-time broadcast TV has increased over last decade by about 40 percent even as viewers have dropped almost 50 percent.

But a drop in teen viewing is a major reason the future looks bleak. Teens spend less than half as much time watching TV as typical adults do. Teens also spend 600 percent more time online. But a lack of online ad supply and the web's generally fragmented nature will keep TV in booming business for the next several years, according to the study.

Tom French, director at McKinsey, said standard reach metrics should not be taken at face value - but on an "adjusted reach" basis.