MENLO PARK, CA: Facebook has admitted a fourth user measurement-related problem in as many months, at the same time as politicians in Europe step up the pressure on the social media giant to address the issue of fake news and to release details of its algorithms.

The latest measurement problem relates to the undercounting of traffic from iPhone users to publishers using Instant Articles, which it has now fixed.

In a blog post, Facebook said it had been alerted to the issue by comScore. "We've since identified this is a result of a recent Facebook update," it said. "This caused an under-reporting of iPhone traffic from Facebook in comScore products between Sep 20 to Nov 30, 2016. iPad and Android traffic were not affected."

Only ten days ago, Facebook revealed it was changing the way it calculates estimates for both the potential overall reach and the estimated daily reach of ad campaigns after it had uncovered errors with those metrics; previously it has also admitted to vastly overestimating the average viewing time for video ads on its platform for two years and to bugs and errors affecting metrics in four products.

That succession of "miscalculated metrics" has brought a strong response from some industry observers but advertisers appear largely unflustered.

"If your mistakes are consistently in your favor, then they aren't mistakes... they're lies," declared Professor Scott Galloway, Adjunct Professor of Marketing at NYU Stern School of Business, as he contrasted the reception Facebook has had with that which would be given the likes of ESPN or IBM if they made similar errors.

Advertisers, however, seem to have reacted with equanimity, with agencies being more outraged.

"This is about leverage," according to Adam Cahill, CEO and founder of programmatic agency Anagram. "The buying community, mainly agencies, see this as a few chinks in Facebook's armor and they're making a bigger deal out of it than it actually is," he told Ad Exchanger.Meanwhile, in Europe, the German government is considering the introduction of a law that would require Facebook – and other platforms – to establish a "legal protection unit" for victims of online hate speech, slander or fake news.

A different angle of attack is emerging in the UK, where Chi Onwurah, shadow minister for industry, noted that "Algorithms aren't above the law".

"The outcomes of algorithms are regulated – the companies which use them have to meet employment law and competition law," she told the Guardian. "The question is, how do we make that regulation effective when we can't see the algorithm?"

Data sourced from Facebook, Ad Exchanger, L2, Financial Times, Guardian; additional content by Warc staff