Facebook advertisers get apology

Facebook Inc. is apologizing to advertisers for what it calls an error that overstated the average length of time users watched videos on the site.

The measurement didn’t affect how much Facebook charges to run video spots, but analysts say ad agencies may have used the Menlo Park, Calif., company’s estimates as a key metric when deciding how much advertising to place.

Facebook executive David Fischer said his company recently discovered its method of calculating the average viewing time didn’t include times when people watched a video for less than three seconds. That made average times seem longer.

He said Facebook has corrected the error, but analysts said it underscores the need for independent verification of such ad metrics.