NEW YORK • Facebook has apologised for an error in the way it measured video viewership, a miscalculation that greatly overstated how much time, on average, its users were spending watching videos.
The incorrect numbers were displayed to partners, including advertisers and publishers, for more than two years, as Facebook aggressively challenged YouTube for online video dominance and urged partners to embrace video publishing and advertising on the platform.
While Facebook on Friday called the problem a "discrepancy", it is a troublesome admission for major advertising agencies and publishers that rely on Facebook's metrics to assess their investments on the platform - whether a 15-second ad or a lengthy video production.
It is also likely to add to advertisers' long-held frustrations with Facebook and other Internet platforms over how closely they guard valuable audience data, and the limits the tech companies place on outside measurement companies.
"About a month ago, we found an error in the way we calculate one of the video metrics on our dashboard - average duration of video viewed," Mr David Fischer, a vice-president of video and marketing partnerships for Facebook, wrote in a post on Friday.
"While this is only one of the many metrics marketers look at, we take any mistake seriously," he wrote. "Our clients' trust and belief in our metrics is essential to us and we have to earn that trust."
Facebook acknowledged in an update to its advertiser support page last month - which largely passed under the radar - that it had been excluding videos watched for less than three seconds from the average viewing times. That greatly inflated the figures presented to partners.
Facebook publicly counts video views after three seconds. By comparison, YouTube counts a view after 30 seconds; Vine, where videos are limited to six seconds, counts after a full video loop.
Advertisers, however, buy Facebook video according to different metrics: either by "impressions", which are measured as soon as a user sees a video in the news feed, or in 10-second views.
"Average duration of video viewed" is not a public metric, nor does it determine how much advertisers pay for video placement. But the figure is displayed prominently in partners' dashboards, and it has helped assuage discomfort about the platform's generous view-counting methods.
In July, Facebook reported blockbuster sales, up 59 per cent from a year ago to US$6.44 billion (S$8.75 billion), and emphasised it was betting on video and video advertising for future growth.
Facebook has made a number of stumbles lately that it has had to scramble to fix.
In May, it grappled with reports that editors of its Trending Topics - which displays some of the most talked-about stories on the social network - were suppressing conservative content. The company later laid off the Trending Topics team.
And this month, the social media giant had to reverse itself after taking down a Pulitzer Prize-winning Vietnam War photo because the picture, which featured a horror-stricken and naked nine-year-old girl, violated the company's content standards for nudity.
Advertisers are unlikely to cut back or change spending on Facebook in a big way because of the metric error, said Mr Brian Wieser, a media industry analyst at Pivotal Research in New York.
But, he said, it could affect "larger brand-focused advertisers who may have been influenced to alter their budget planning" based on Facebook's previous data.