Facebook on Tuesday said its guidelines for deciding what stories are cited as trending do not allow "the prioritization" of one viewpoint over another.

Facebook was responding to an accusation that its Trending Topics section routinely suppresses news stories that would be of interest to conservative readers.

The accusation initially was made by the conservative news portal Breitbart News last week and was repeated on Monday at the website Gizmodo, which quoted a former journalist who worked on Facebook's Trending Topics project.

The source for the Gizmodo story, who was not identified, said stories about the right-wing gathering of the Conservative Political Action Conference in Washington in March, reports about conservative politicians Mitt Romney and Rand Paul and other conservative topics were barred from the section despite their popularity among Facebook users.

Facebook's Tom Stocky, who is responsible for Trending Topics, said in his blog that the guidelines for selecting topics "do not permit the suppression of political perspectives."

Stocky said popular topics are initially chosen by an algorithm, then audited by a review team to confirm that the topics are in fact trending news. The review team members operate under what Stocky said were "rigorous guidelines."

"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," his blog post said.

The Gizmodo story said several former reviewers claimed they were instructed to artificially "inject" selected stories into the trending news section even if they were not popular enough to warrant inclusion or in some cases were not trending at all.

The former reviewers, all of whom worked as contractors, also said they were directed not to include news about Facebook itself in the trending module, Gizmodo said.

Stocky's blog post didn't address that accusation, but did say that the guidelines for Trending Topics were under constant review.

Facebook - no longer a platform centered on cute cat photos and exhcanges between hobbyists - has become increasingly important as a source of news. About two-thirds of Facebook users (63 per cent) get news at the portal, according to the US pollster Pew Research.

About half the Facebook users surveyed by Pew said they get information about the US presidential race from Facebook.

The conservative news portal Breitbart News, in the initial story reported last week, also quoted a former reviewer as saying Facebook’s Trending Topics section purposely suppressed content from "blacklisted outlets," including Breitbart News.

The website said this was the case despite its position in the top 25 most engaged publications on the social network for six months in a row.

White House spokesman Josh Earnest was asked about the controversy during Tuesday's briefing. He said the White House was pleased to see the Facebook statement saying it didn't engaged in such activity.

Earnest said the current media environment "makes it easy for any individual to surround themselves with voices and opinions that they share" and there is a "special burden on consumers to seek out opinions that differ from theirs."

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