Facebook Inflated Video Ad Viewing Stats for 2 YearsThe error has been fixed and did not impact billing, the social network said.

Facebook has been overestimating average video ad viewing times for two years,The Wall Street Journalreports, a revelation not likely to sit well with advertisers or those who have shifted resources to video to take advantage of the trend.

In apostlast month on the Advertiser Help Center site, Facebook disclosed an error in the way it was tallying video views. It had told advertisers that views were calculated by taking the total time spent watching a video divided by the total number of people who had played that video. In reality, it was taking the total time spent watching a video and dividing it by the number of people who had viewed it for three seconds or more.

According to theJournal, advertisers asked for more information and came to find out that average time spent watching videos was overestimated by as much as 60 to 80 percent.

"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," a Facebook spokeswoman toldPCMag. "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."

YouTube star Hank Greencomplained about Facebook's video metricsmore than a year ago, when he called out Facebook for being dishonest about how it runs its video business.

"YouTube … counts views in a logical way," he wrote in an August 2015 blog post. "The view is counted at the point at which people seem to actually be engaging with the video and not just immediately clicking away." This is important, he pointed out, because Facebook can inflate its video view numbers to sell ads.

On Twitter last night,Green said"this news about Facebook over-estimating average advertisement watch time is totally separate from them over-estimating viewership." But it's still a 'huge' issue as "those ad dollars went to Facebook fraudulently," he writes.

"We apologize for the inconvenience and we hope that these new metrics, in addition to other video metrics in reporting will help you better evaluate your ad performance," the Advertiser Help Center said.