From the Wall Street Journal:
Several weeks ago, Facebook disclosed in a post on its “Advertiser Help Center” that its metric for the average time users spent watching videos was artificially inflated because it was only factoring in video views of more than three seconds. The company said it was introducing a new metric to fix the problem.
A classic case of left censoring (in this case, by “accident”).
Also this:
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%, according to a late August letter Publicis Media sent to clients that was reviewed by The Wall Street Journal.
What does this information tell us about the actual time spent watching Facebook videos?