Facebook (NASDAQ: FB) depends on its editorial team to curate news for its trending module, according to leaked internal documents.

The Guardian reported that the social media giant's editors are expected to make decisions about what is displayed in the trending news feed. In a subsequent statement, Facebook clarified that topics are "surfaced by algorithm" and then reviewed by its editorial team.

Recently the review process for Facebook's trending topics — that little list of newsy links that everybody sees on the upper right side of their feeds — came under fire for allegedly suppressing conservative news stories. According to Facebook's statement and documents obtained by The Guardian, a story has to have already been covered by at least five major media outlets, including CNBC's sister outlet NBC News, in order to be considered "a national story."

How Facebook chooses the topics for the popular trending section matters because so many people trust it as a primary news source.

About 63 percent of Facebook users said that they use the platform as a source of news for "events and issues outside the realm of friends and family," according to a Pew Research study performed in partnership with the James L. Knight Foundation last summer. A subsequent study found that Facebook drove 82 percent to 84 percent of social traffic to longer and shorter stories, respectively.

Facebook told CNBC on Thursday that The Guardian had an outdated version of its editorial guidelines and that news topics could not be added if they did not already belong there. The company said that its current guidelines state: "The editorial team CANNOT inject a newsworthy topic if it is not appearing in the demo or review tools. We will track these instances so the engineers can fix for the future."

Justin Osofsky, vice president of global operations at Facebook, said that, if anything, the guidelines demonstrate the company's commitment to fairness.

"Facebook does not allow or advise our reviewers to systematically discriminate against sources of any political origin, period," Osofsky told CNBC in an email.

"What these guidelines show is that we've approached this responsibly and with the goal of creating a high-quality product — in the hopes of delivering a meaningful experience for the people who use our service," he said.

Osofsky added that Facebook relies on more than 1,000 news sources from around the world to attempt to verify and characterize world events and topics.

"The intent of verifying against news outlets is to surface topics that are meaningful to people and newsworthy," Osofsky said. "We have at no time sought to weight any one viewpoint over another, and, in fact, our guidelines are designed with the intent to make sure we do not do so."

The Guardian noted that Facebook moved away from a purely algorithmic curation of its trending topics when the company was criticized in 2014 for inadequately weighting stories about events in Ferguson, Missouri.

Read the full report in The Guardian.

Disclosure: NBC News is owned by CNBC's parent company, NBCUniversal. CNBC also has content-sharing partnerships with a number of the outlets that Facebook reportedly uses to determine how to weight a story.

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