"Our general view is marketers shouldn't ever pay for impressions that are never seen by users," Brad Smallwood, Facebook's vice president of marketing science, told CMO Today.
But where the MRC defines a viewable ad as one where at least half of it can be seen on screen for at least one second, Facebook takes a less rigorous approach, classifying an ad as visible the moment any part of it appears on screen.
"As soon as an ad comes into view it starts creating value for the advertiser," Smallwood stated.
"Some people say they just ignore advertising, but even the act of ignoring advertising can still create value for an advertiser," he added. "That's the rationale behind our using the moment of viewing as the measure."
Not all advertisers may agree – Unilever, for example, insists on 100% viewability – but Facebook said it was working with the MRC and a consortium of advertisers and agencies to "develop more robust standards for viewable impressions".
Imperfect though it may be, some advertisers have welcomed Facebook's stance. "Adherence to viewable impressions is one of the reasons we count Facebook as a primary media partner," said Brandon Rhoten, vp/digital & social media at restaurant chain Wendy's.
"They're willing to hold themselves to a standard we can both agree is acceptable," he told Ad Exchanger, "and they're actually trying to get ad dollars from our TV budget rather than our digital budget."
Smallwood argued that the industry's focus on what percentage of an ad should be visible or how long a video needed to play had been unhelpful.
"Let's agree that impressions need to be viewable and then we'll deal with the second issue, which will be more about creating guidelines than creating an accounting mechanism."
Data sourced from Facebook, Ad Exchanger, Wall Street Journal; additional content by Warc staff