Meanwhile, a growing cast of shows on cable networks are siphoning away a bigger share of the shrinking number who are tuning in overall.

WSJ: No Longer 'Must-See TV'

But Facebook argues that for brand advertisers, fine-tuning the number of times a particular consumer sees an ad as well as ensuring that the ad has reached all of its target audience are far more effective techniques.

FORBES: Facebook: Ad Clicks Show We're Bad Value So Let's Measure Advertising On Facebook Another Way