A report by Gizmodo said staff who aggregated content for Facebook were encouraged to ignore news from fringe conservative sites such as Breitbart until it appeared on more mainstream sites such as CNN.

Tom Stocky, the company’s vice president of search, wrote in a Facebook post that “we take these reports extremely seriously, and have found no evidence that the anonymous allegations are true”. He added that what was being alleged would be not be “technically feasible”.

He wrote: “We have in place strict guidelines for our trending topic reviewers as they audit topics surfaced algorithmically. Reviewers are required to accept topics that reflect real world events, and are instructed to disregard junk or duplicate topics, hoaxes or subjects with insufficient sources. 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.”

As well as reporting that Facebook discriminates against conservative sites, Gizmodo alleged that Facebook manipulates trends to ensure that weighty news appears on the list even when there’s little conversation around it. One example given was Black Lives Matter, the American civil rights movement. “Facebook got a lot of pressure about not having a trending topic for Black Lives Matter,” a former aggregator told Gizmodo, so “they realised it was a problem, and they boosted it in the ordering. They gave it preference over other topics.”

Stocky also denied that report. “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.”

After the initial report, editors of the conservative sites expressed their outrage at the suggestion they had been overlooked by Facebook’s aggregators.

Breitbart News’s editor in chief, Alex Marlow, said the report confirmed “what conservatives have long suspected: Facebook’s trending news artificially mutes conservatives and amplifies progressives”.

Marlow added: “Facebook claims its algorithm simply populates ‘topics that have recently become popular on Facebook’ in its trending news section, but now we know that’s not true.”

Hugo Gurdon, the Washington Examiner’s editorial director, said: “We’ve read the report, and it is obviously of considerable interest to us. We plan on working with Facebook to ensure Washington Examiner’s valuable content gets the attention it obviously deserves.”

It’s the second time in a week that Facebook has had to answer difficult questions about how its trending topics work. A previous report by Gizmodo alleged that the journalists who work on the trending product were never explicitly told to avoid outlets like Breitbart, The Blaze or World Star Hip Hop, but would regularly avoid the sites anyway. Similarly, they were discouraged from mentioning the rival social network Twitter in their summaries, instead referring to “social media”.

But at least one story which might previously have been swept under the carpet was trending on Facebook on Tuesday morning: under the headline “Facebook Trending” was a link to Gizmodo’s original news story.

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