Facebook admits, for the fourth time, that it messed up its measurement metrics

Facebook announced on Friday it had made another measurement error, this time affecting publishers that use its Instant Articles content platform.

It marks the fourth time in recent months Facebook has admitted to overstating or understating the metrics that publishers or advertisers use to measure the effectiveness of their activity on the platform.

In the latest disclosure, Facebook said comScore, a third-party measurement firm it partners with, found the social network was undercounting traffic from iPhone users to their Instant Articles content.

Instant Articles is a platform Facebook launched last year where publishers upload their content directly to Facebook in order to help it load faster than a standard web page. Publishers can sell their own ads and keep 100% of the proceeds or get Facebook to sell the ad slots for them and allow the social network to take a 30% cut of that revenue.

Facebook said in an update to a previous blog post that a recent software update led to iPhone traffic being undercounted between September 30 and November 30 this year. It has now fixed the error, it added.

The Wall Street Journal, which first reported Facebook's latest measurement miscalculation, spoke to a person familiar with the matter who suggested the issue affected some publishers' traffic numbers by between 10% to 20%. For most publishers, however, the error affected less than 1% of their traffic, the source said.

Facebook began being more upfront about metrics miscalculations in November, when it announced the launch of a new "Metrics FYI" blog series to provide updates on when it discovers errors and bugs. Two months earlier, Facebook's ad boss Carolyn Everson admitted the company could have done better to communicate publicly about a previous video metrics error , which most people found out about via a Wall Street Journal article.

The Metrics FYI blog has now published three separate announcements about measurement errors. First, a series of different metrics errors announced when the blog first launched; then an announcement on December 9 that it had miscalculated metrics on its Graph API for advertisers and the number of likes and reaction emojis page owners see for their videos; and now this Instant Articles error.

In all of these cases, none of the errors had any effect on the amount advertisers were charged for Facebook advertising.

Facebook will be hoping that its upfront communication about software bugs and measurement issues will reassure advertisers they can trust the data they are receiving from the social network. However, the negative headlines could also raise concerns among advertisers and publishers that the Facebook measurement data they plan their social media activity around has the potential to be inaccurate.