Facebook has been artificially inflating a key video performance metric by between 60 and 80 per cent, according to a letter sent by Publicis Media to its clients, with figures from the past two years affected.

According to the Wall Street Journal, which reviewed the letter, the discovery was prompted by an answer given by Facebook on its Advertiser Help Centre several weeks ago, which caused several ad agency executives to investigate and push Facebook for more details.

The post revealed that Facebooks metric for the average time users spent watching videos only factored in video views of more than three seconds, eliminating a huge number of very brief views.

While Facebook has since introduced a new metric to fix the problem, the earlier method overestimated average time spent watching videos by up to 80 per cent, according to the social network.

“We recently discovered an error in the way we calculate one of our video metrics,” a Facebook spokesperson said in a statement. “This error has been fixed, it did not impact billing, and we have notified our partners both through our product dashboards and via sales and publisher outreach. We also renamed the metric to make it clearer what we measure. This metric is one of many our partners use to assess their video campaigns.”

The news is an embarrassing oversight for Facebook, which has been pushing its transformation into a video-heavy platform for the past few years, and will impact advertiser trust with the firm, with the skewed metric possibly having impacted spending with the company.

The inflated metric also impacts publishers and media companies using Facebook to distribute video content, which in turn may have informed their decision making.

“As Martin Sorrell expressed at WPPs recent earnings call, its time for better measurement, not just offline but online too,” said Theo Theodorou, managing director for EMEA at xAd. “This case of inaccurate viewability reporting shows that we cant have adtech players marking their own homework anymore.

“For a true and fair assessment of viewability, engagement and attribution, having third parties objectively qualifying the insights brands are getting is not only preferable, but is a strategic imperative for creating effective marketing strategies.”