On the cusp of Advertising Week, Facebook has some egg on its face when it comes to perhaps the industry's most promising space—digital video.

The Wall Street Journal reported on Thursday evening that ad agencies were upset with Facebook for giving them an inflated metric for the average amount of time users spend watching clips on the social platform. The publication cited Publicis Media, which was informed by Facebook that the social network's measurement system had previously overestimated the average time spent viewing videos by anywhere between 60 percent and 80 percent. The Journal gained access to a letter sent to clients from Publicis in late August.

The inflation occurred because Facebook—when calculating average time spent—was only factoring in video views of more than three seconds rather than lesser view times. This faulty practice went on for two years, said The Journal.

UPDATE: Facebook authored a blog post today to futher explain the metric-based miscue. David Fischer, vp of advertising and global operations, wrote: "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. The metric should have reflected the total time spent watching a video divided by the total number of people who played the video. But it didn't – it reflected the total time spent watching a video divided by only the number of 'views' of a video (that is, when the video was watched for three or more seconds). And so the miscalculation overstated this metric. While this is only one of the many metrics marketers look at, we take any mistake seriously."

At any rate, the stat in question may have influenced how much some brands have been spending on Facebook, which made $6.24 billion in ad revenue in Q2. At the same time, Facebook has incredible reach, approaching 2 billion monthly global users, so it's possible the bad data point wouldn't have affected marketers' spend in dramatic fashion.

It's hard to say, exactly, but Facebook said they have corrected the situation. The company revealed in late August that its average-time-viewed calculations have been too high in a post on its Ads Help Center hub before being pressed by ad buyers about the specifics, according to The Journal.

"We recently discovered an error in the way we calculate one of our video metrics," the company said in a statement emailed to Adweek. "This error has been fixed, it did not impact billing, and we have notified many of 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."