The social network said in a Newsroom post that the bug only incorrectly tallied smartphone users on their mobile browsers, and not desktop users or those on the Facebook mobile application.
Facebook described the bug as follows:
The bug affected billing only for the following conditions: for the video carousel ad unit; when the advertiser chose to bid on link clicks; and only for people who were on smartphone web browsers. In these cases, instead of being billed only for link clicks (clicks to an advertiser’s selected destination), these advertisers were incorrectly billed when people clicked on the videos in the carousel to enlarge and watch them. Advertisers will receive a full credit for the charges they incurred for these misattributed clicks.
The social network added that the impact of this bug was minimal, as most users access Facebook via its app, meaning that impressions from mobile web browsers account for a small percentage of overall ad impressions. The impact from a billing perspective was 0.04 percent of ad impressions, according to Facebook.