Facebook is the biggest driver of stories to news sites in the world. But does every story have an equal chance of “trending” on Facebook?

According to a Gizmodo investigation that’s spoken to numerous former “news curators” at the social network, the social media giant “routinely suppressed news stories of interest to conservative readers from the network’s influential ‘trending’ news section”.

Stories about Mitt Romney, Rand Paul and right-wing gatherings in America trended on the site organically but suffered from this suppression, according to Gizmodo. Meanwhile, stories on “worthy” causes like Syria were artificially injected into trending stories on the site in a bid to make Facebook’s userbase appear more high-brow. If something was trending on Twitter, famous for breaking news, but yet to make an impact on Facebook, news curators would often artificially insert that news into the trending stories. Popular articles about Facebook itself, former curators added, wouldn’t make the trending section without approval through numerous layers of management. Gizmodo’s analysis:

“Facebook’s news section operates like a traditional newsroom, reflecting the biases of its workers and the institutional imperatives of the corporation. Imposing human editorial values onto the lists of topics an algorithm spits out is by no means a bad thing—but it is in stark contrast to the company’s claims that the trending module simply lists ‘topics that have recently become popular on Facebook.’

A Facebook response to the story says there are “rigorous guidelines in place … to ensure consistency and neutrality” when it comes to trending news.

“We take allegations of bias very seriously. Facebook is a platform for people and perspectives from across the political spectrum … [G]uidelines 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.”