The last few years have been a story of strong success for Facebook’s advertising business, but its growth in video has hit a problem. The social network has admitted that it had been overestimating average viewing time for videos on its service for two years. The Wall Street Journal has been talking to advertising agencies briefed on the issue. “Ad buying agency Publicis Media was told by Facebook that the earlier counting method likely overestimated average time spent watching videos by between 60% and 80%,” reported the Journal. Facebook says that the 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” while pointing out that other metrics are available to help people measure campaigns.