Name a metric, any metric, for measuring audience attention, and there is (a) a reason why it's useful; (b) a reason why it's worthless; and (c) a way for digital media companies to corrupt it.
Page views (e.g.: clicks) used to be the most common currency of online attention, only to be replaced by unique visitors (e.g.: readers). But in the viral age, "readers" doesn't mean what it used to mean. If you were a newspaper in the 1980s, readers = subscribers who receive your bundle of paper each morning. In a bookmarked-site world, readers meant Web visitors who drop by a few days a month at most. But in the viral age, readers can mean 1 million Facebook users who see some sensational headline, clicked it, and scurried away, having no recollection of what URL hosted the article, and never visit the site again.
Reader used to be a person. Now it's a spectrum. There are dedicated readers on one end, Tsetse-fly-brained Facebook browsers on the other, and fully engaged one-time readers in the middle. Surely they shouldn't count equally to an advertiser seeking an consistently engaged and knowable audience.
That's why some digital companies are trying to the Internet less like a newspaper and more like TV. YouTube now measures “Time Watched." Medium counts “Total Time Reading." Chartbeat tracks “Average Engaged Time.” And now Upworthy—the viral firehose of the Web—announced that it's developed a new metric. "Attention minutes" seeks to measure time spent watching or reading an Upworthy page, by studying "length of time a browser tab has been open, how long a video player has been running, and the movement of the mouse on screen," according to Nieman Lab.