Jessica Guynn

USA TODAY

SAN FRANCISCO — Is Facebook, one of the world's most powerful distributors of news on the Internet, censoring politically conservative articles?

Facebook denies it, but that's the allegation in a published report that set off a firestorm among conservative media on Monday.

According to tech news outlet Gizmodo, a former Facebook news curator says popular articles from politically conservative outlets on conservative subjects were deliberately left off of the "trending news" sidebar. Among the allegedly verboten topics: Wisconsin Gov. Scott Walker and former Fox News contributor Steven Crowder.

“I’d come on shift and I’d discover that CPAC [Conservative Political Action Conference] or Mitt Romney or Glenn Beck or popular conservative topics wouldn’t be trending because either the curator didn’t recognize the news topic or it was like they had a bias against Ted Cruz,” the former news curator told Gizmodo.

Facebook denied the report, saying it has "rigorous guidelines" to ensure political neutrality in the distribution of news to its 1.6 billion users around the globe.

"We take allegations of bias very seriously. Facebook is a platform for people and perspectives from across the political spectrum. Trending Topics shows you the popular topics and hashtags that are being talked about on Facebook," the company said in an emailed statement.

"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."

The Gizmodo report underscored rising alarm about Facebook's growing influence in what news gets read. At issue: the small box in the upper right of a Facebook page that lists news topics that are popular on Facebook. Facebook relies on automated systems to identify what's popular. A team of curators then refine the list.

Breitbart News editor in chief Alex Marlow told the Guardian the Gizmodo report confirmed "what conservatives have long suspected: Facebook’s trending news artificially mutes conservatives and amplifies progressives." The Drudge Report ran a photograph of Facebook COO Sheryl Sandberg: "Not Leaning In ... Leaning Left!"

Gizmodo says the former news curator, who is politically conservative, asked for anonymity, citing fear of retribution from Facebook.

One former Facebook news curator, also unnamed, challenged the Gizmodo report, telling the Guardian that newsworthiness was judged by how often an article appeared in trusted news outlets such as the New York Times and the BBC. This person did not see any evidence of conservative news being suppressed.

"What we have from the report is anecdotal and it’s tough to know what’s going on," wrote TechCrunch's Josh Constine. "Facebook’s biggest problem may have been not realizing humans have biases – including political ones."