Facebook CEO Mark Zuckerberg. Justin Sullivan/Getty Images Facebook exaggerated a key metric advertisers use to assess the performance of their videos on the platform by potentially as much as 80% for more than two years, The Wall Street Journal reported on Thursday.

It's an embarrassing screwup, and the report suggests one ad buyer, Publicis Media, was "upset" by the error. But while it certainly looks bad, there are a few reasons it why probably won't be as bad for Facebook as it seems.

The company acknowledged in a post on its Advertiser Help Center website that its metric for average video viewing time had been inflated because it didn't include video views lasting under three seconds. Facebook added that it was introducing a fix to provide a more accurate metric to include all views: "Average Watch Time."

A Facebook representative told Business Insider: "We recently discovered an error in the way we calculate one of our video metrics. 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."

Why this isn't as bad as it looks

Average video view time is just one metric Facebook offers advertisers to assess the impact of their videos. Many marketers look at total views or the view completion rate to determine whether their videos were a success.

Crucially, average video view time isn't what Facebook calls a "billable event." Facebook doesn't charge for video ads that were viewed for less than three seconds, unless advertisers actively choose to be charged on this basis. You can't bid on "average view time."

So many marketers may have simply assumed — as advertising executives including Anheuser-Busch InBev senior director of digital connections Azania Andrews and Laundry Service CEO Jason Stein have pointed out on Twitter — that people who watched a video for less than three seconds (therefore users they didn't pay to reach) were not included in the average view time metric anyway.

Why it's still pretty bad

Regardless, it's still not a good look for Facebook. It continues the narrative that Facebook is a black box that doesn't expose its inner workings to the people who fund its very existence.

WPP CEO Sir Martin Sorrell. WPA Pool/Getty Images Sir Martin Sorrell, the CEO of the world's biggest advertising group, WPP, which spent $1 billion of its clients' marketing budgets on Facebook last year, has called Facebook out on this before. He has previously said Facebook's three-second video viewability threshold was "ludicrous" and on several occasions has called for Facebook to offer better independent measurement tools.

He told Business Insider on Thursday: "That's why we invested in comScore and built a media measurement business in over 50 countries around the world. We have also been calling for a long time for media owners like Facebook and Google not to mark their own homework and release data to comScore to enable independent evaluation. The referee and player cannot be the same person."

Keith Weed, the chief marketing officer at the consumer-goods giant Unilever, said in an interview last year that without online media giants offering third-party verification, "It's like letting them mark their own homework."

In its defense, Facebook has formed a partnership with the third-party analytics firm Moat to provide data on how its Facebook video ads are performing — a move that Weed in a Facebook press release said was "very encouraging." The company has since opened up its platform to other video-measurement partners, including Integral Ad Science, Nielsen, and comScore. Marketers can request this data, though it doesn't come as standard.

While some people might argue that, regardless, many agencies may have been using Facebook's average video view data to encourage marketers to spend on Facebook's video ads, it's up to marketers to be savvy and use all the data available to them — particularly independent, third-party data — to determine whether those ads are performing well.

After this article was first published on Friday, Facebook's vice president of advertising and global operations, published this statement in an attempt to clarify the issue: