SAN FRANCISCO (NYTIMES) - One of Facebook's biggest appeals to advertisers is its wealth of data on the millions of people who use the site daily. Yet the social network recently has come under scrutiny from some of its biggest advertisers for errors in that data.

On Friday (Dec 9), Facebook announced it was fixing three of its ad-measurement tools after it found that several methods it used to describe audience numbers to advertisers were inaccurate.

The disclosure follows other mistakes in advertising measurement from Facebook, a setback for a company that has long prided itself on its sophisticated advertising technology.

In September, Facebook apologised for overstating how it measured video viewership, an important metric for marketers, and in November, the company again noted errors across four of its products.

This time, Facebook said it had inaccurately measured the number of people an advertiser had the potential to reach across the social network. The company is retooling the way it displays reactions to live videos, and is looking into incorrect displays of the number of likes or shares a Facebook post has received when someone is searching for it on a mobile device.

The errors are not monumental, the company said, and do not involve metrics used to bill advertisers.

In November, Facebook said it would explore more third-party verification of its numbers by working with outside partners. The company will also establish a "measurement council" with trusted partners, something it will provide more details on in the future.

Still, the missteps have given pause to some advertisers and ad buying agencies, many of which have moved more spending to Facebook in the past five years.

Over the first half of 2016, marketers spent US$5.7 billion (S$8.2 billion) on Facebook advertising in the United States, which represented 43 per cent of digital advertising growth domestically, according to estimates from the Interactive Advertising Bureau.

Publicis Media, a major digital advertising buyer, advised clients of the errors in September and called for greater outside oversight of Facebook's measurement practices.

In the months since, Facebook has tried to divulge more information to outsiders in a company blog called Metrics FYI.

"We know how important it is to be open about meaningful updates we make to our metrics," Facebook said in its most recent post.