Report: Facebook manipulates what's "trending"

But a new report on the technology website Gizmodo suggests that the list of top trending topics is not quite as objective as it appears to be. Idiosyncrasies, institutional rules, and personal biases may all come into play in determining what makes the list.

According to a source cited by Gizmodo, Facebook's team of news "curators" routinely excludes trending stories from conservative media sites from the trending section.

A Facebook spokesperson tells CBS News it takes these allegations of bias "very seriously," but the company insists "we... have found no evidence that the anonymous allegations are true."

The Gizmodo source, a former Facebook curator who identifies as politically conservative, said the team shied away from featuring conservative-interest stories even when they qualified as "trending" using Facebook's internal algorithm, unless they were picked up by another mainstream news organization. Subjects that did not make the cut included Mitt Romney, Rand Paul, CPAC, Wisconsin Gov. Scott Walker, the Drudge Report, and former Navy SEAL Chris Kyle, according to Gizmodo. "I believe it had a chilling effect on conservative news," the ex-curator told the website.

The omissions were the result of curators' personal news judgment, not any mandated policy from Facebook, the source told Gizmodo. Last week, Gizmodo reported that Facebook's trending news team is primarily run by 20- and 30-somethings who graduated from Ivy League and private East Coast schools.

A new report sheds light on how Facebook produces its highly influential "trending" section. Facebook/CBS News

However, Facebook says it does not suppress political views and has guidelines in place to keep the system from being improperly manipulated.

"Facebook is a platform for people and perspectives from across the political spectrum," a Facebook spokesperson told CBS News. "Trending Topics shows you the popular topics and hashtags that are being talked about on Facebook. There are rigorous guidelines in place for the review team to ensure consistency and neutrality. These guidelines do not permit the suppression of political perspectives. Nor do they permit the prioritization of one viewpoint over another or one news outlet over another. These guidelines do not prohibit any news outlet from appearing in Trending Topics."

Beyond red versus blue, curators were trained to "inject" select stories into the trending module even when they weren't trending, according to sources quoted by Gizmodo. Sometimes, after a non-trending story was "injected" into the trending module, it eventually became the number one trending news topic on Facebook -- a startling reminder of Facebook employees' power to shape public consciousness.

According to Gizmodo, topics that were artificially injected into Facebook's trending section included the disappearance of Malaysia Airlines flight MH370, the Charlie Hebdo attacks, and Black Lives Matter.

However, Facebook denies that claim. In a statement, Tom Stocky, Facebook's vice president of search, said: "There have been other anonymous allegations -- for instance that we artificially forced ‪#‎BlackLivesMatter‬ to trend. We looked into that charge and found that it is untrue. We do not insert stories artificially into trending topics, and do not instruct our reviewers to do so."

In a Facebook post, he also stressed, "Facebook does not allow or advise our reviewers to systematically discriminate against sources of any ideological origin and we've designed our tools to make that technically not feasible. At the same time, our reviewers' actions are logged and reviewed, and violating our guidelines is a fireable offense."

Former employees say they were instructed not to include news about Facebook into the trending section even when Facebook itself was trending, Gizmodo reported. "When it was a story about the company, we were told not to touch it," Gizmodo quoted a source as saying. "It had to be cleared through several channels, even if it was being shared quite a bit. We were told that we should not be putting it on the trending tool."

This account challenges the perception of Facebook's trending section as an objective, algorithm-powered pipeline for information. "In other words, Facebook's news section operates like a traditional newsroom, reflecting the biases of its workers and the institutional imperatives of the corporation," Gizmodo wrote.

According to Facebook, the highly visible trending module simply reflects "topics that have recently become popular on Facebook," based on factors like "engagement, timeliness, Pages you've liked and your location."

As of March 2016, Facebook had an average of 1.09 billion daily active users. In its mission statement, the company prides itself on helping users "discover what's going on in the world."