Facebook hugely overestimated the average viewing time for video ads on the social media site for two years, the Wall Street Journal reported.
According to the WSJ, Facebook told major ad agencies and ad buyers that the way Facebook had counted viewing time had artificially inflated the average viewing time by between 60 percent to 80 percent, because it only counted video views of more than three seconds. Facebook fixed the issue several weeks ago, the report said.
A Facebook spokesperson told CNBC in a statement that the error in one of its video metrics did not impact the way it billed advertisers, and that it had communicated the issue to many advertisers both through its "product dashboards" as well as by contacting them.
"This metric is one of many our partners use to assess their video campaigns," the spokesperson said.
To read the full WSJ report, click here.
Facebook is a formidable force in social media advertising, with eMarketer estimating that the platform would take almost 68 percent of all social media advertising spend worldwide in 2015.
Advertising revenue is Facebook's main revenue stream. The social network reported ad revenue of $6.24 billion in the second quarter, beating the $5.8 billion expected by analysts surveyed by StreetAccount.
Facebook did not immediately respond to CNBC's request for comment on the WSJ's report.