Mark Zuckerberg said Facebook is investigating claims of censoring conservative news reports, following a discussion in the US Senate. Facebook's Trending Topics has been criticised after anonymous former editors revealed that popular conservative news topics were not given the prominence they were due. A copy of Facebook's "Trending Review Guidelines" shows how Facebook trained it's internal employees to curate Facebook's trending section to "inject" and "blacklist" topics.

One former anonymous employee told tech news website Gizmodo that Facebook "used external topics, to push more external topics into people's feeds". The team of curators had the power to vet topics coming from a series of sources, including organisations such as BBC, the Guardian, NBC News, CNN and Buzzfeed – most of which generally reflect centre-left views – which may lead to the lack of attention to conservative topics, which featured sources such as Fox News.

Curators would regularly avoid sites such as World Star Hip Hop and The Blaze, although they were not explicitly told to. They also had the power to "deactivate" a trending topic, if at least three mainstream news sources were not covering it.

Facebook's curators also admitted to artificially inflating the popularity of "Black Lives Matter" by placing it among Facebook's Trending Topics.

"Facebook got a lot of pressure about not having a trending topic for Black Lives Matter," an ex-curator told Gizmodo. "They realised it was a problem, and they boosted it in the ordering. They gave it preference over other topics."

The curators, who are not direct employees of Facebook, are contracted through companies such as Pro Unlimited and BCForward, who are then subcontracted by Accenture. Facebook introduced the Trending feature on the social media platform in 2014 to show users popular topics, after realising important news stories, such as the Ferguson Protests in 2015, were being overlooked by the algorithms used to automatically generate trending topics.

Facebook's vice-president of operations Justin Osofsky has said that the tweaks only happen after algorithms identify a spike in a topic gaining popularity. "One of the most powerful ways people connect is around major events — in their communities and in the larger world," he wrote 12 May, on Facebook's Newsroom mini-site. "At its core, Trending Topics is designed to help people discover major events and meaningful conversations… to help people discover content that is both popular in the world and meaningful to them."

On the same day, Mark Zuckerberg posted that he will be investigating allegations that Trending section of Facebook is biased.