Well that’s not good. In a letter to the ad-buying agency Publicis Media, which was obtained by the Wall Street Journal, Facebook admitted to misstating its assessment of how long, on average, viewers watch videos on Facebook. Facebook said that, when measuring average view time, it had only been taking video views of longer than three seconds into account. Facebook now acknowledges that this was a mistake, and the company says that it has begun taking all video views into account when calculating average view time. According to Facebook’s letter to Publicis, the social media giant’s earlier method of measuring video views likely inflated its assessment of average view time by 60 to 80 percent. (Yikes.) As the Journal points out, it’s entirely possible that Facebook’s overly rosy metrics have misled advertisers and agencies as to just how many viewers have been seeing their ads. And that’s exactly the kind of flawed information that can sway buyers into buying on Facebook instead of, say, YouTube or other publishers. Facebook insists that average view time is merely one of many video viewing metrics it uses, and that the “error…did not impact billing.” It remains to be seen whether buyers will adjust their spends in response to the news.