SAN FRANCISCO: Facebook is under fire again after the social media giant discovered bugs and errors affecting metrics in four products: Page Insights, its video product, Instant Articles and referrals in Analytics for Apps.

According to a release from Facebook, reach statistics – which show the number of visitors to posts – may have counted repeat visitors more than once. The bug has been live since May.

Facebook said that the bug did not affect metrics on paid advertisements, but as organic reach dwindles to near zero, marketers have every reason to be concerned.

Once the affected metrics are de-duplicated, seven-day summaries in the overview dashboard will be up to 33% lower on average and the 28-day view will be a huge 55% lower.

For Facebook, which relies on its enormous user base to attract advertisers, the discrepancy is just one more issue after several months of criticism.

An earlier bungle on video metrics angered marketers, who have invested heavily in the platform's online video offerings as it has become more popular.

While Facebook is endeavouring to fix the problems as soon as possible, many marketers are increasingly concerned about the reliability and transparency of measurement on the social media site and the latest measurement error comes as Facebook also faces heat on its ability to manage viral fake news.

According to a new analysis by Buzzfeed News, the 20 top-performing false election stories from fake "news" sites and blogs generated 8,711,000 shares, reactions, and comments on Facebook.

Comparatively, the 20 best-performing election stories from 19 major news outlets, including the New York Times and NBC News, generated a total of 7,367,000 shares, reactions, and comments on Facebook in the last three months of the US presidential race.

Facebook said it will put several new measures in place to provide piece of mind on measurement for clients, including more third-party verification.

For example, the company is partnering with Nielsen to include Facebook video and Facebook Live viewership in Nielsen's Digital Content Ratings (DCR).

Data sourced from Facebook, BuzzFeed News; additional content by Warc staff