On Monday, the website Gizmodo reported that certain topics had been “blacklisted” from Facebook’s trending news, according to an unnamed source who had worked as a curator at Facebook. “I believe it had a chilling effect on conservative news,” the individual said.

The backlash was as swift as it was furious. On its blog, the Republican National Committee declared that Facebook “must answer for its conservative censorship.” Sen. John Thune (R-S.D.) sent a letter to Zuckerberg on Tuesday, asking the 31-year-old tech entrepreneur to explain what goes on in the Trending Topics section.

Previously, Facebook had said its trending news module was rooted almost entirely in algorithms. Although algorithms can reflect biases, intentional or not (code is, after all, a human creation) this explanation lent an air of computational objectivity to the trending topics section.

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On Thursday, however, Facebook elaborated upon the role humans play when selecting Trending Topics. Potential news is “surfaced by an algorithm,” but human curators may combine related topics into one item. Or the curators weed out certain non-newsworthy items that happen to be trending — Facebook gave the example of “#lunch,” which spikes around various global lunchtimes, as a hashtag booted from the Trending Topics section. The news algorithm scans for surging stories on Facebook, in addition to the RSS feeds of websites as varied as the Wall Street Journal, ViralNova and The Washington Post.

Facebook has 1.6 billion members, and is one of the most significant drivers of internet traffic to media websites. But how much influence, exactly, the Trending Topics section has is unclear. The Gizmodo report did not address the News Feed, the personalized list of stories users see when they log into Facebook. Facebook has also denied that any news was blacklisted due to a conservative viewpoint. “We have found no evidence that this report is true,” Zuckerberg wrote Thursday. “If we find anything against our principles, you have my commitment that we will take additional steps to address it.”

“We have rigorous guidelines that do not permit the prioritization of one viewpoint over another or the suppression of political perspectives,” he added.

There are no details yet about whom Zuckerberg might meet, and Facebook was unable to respond immediately to an inquiry from The Washington Post. “Facebook stands for giving everyone a voice,” Zuckerberg wrote Thursday night. “We believe the world is better when people from different backgrounds and with different ideas all have the power to share their thoughts and experiences.”