Facebook this week published a blog post in its newsroom in response to recent accusations that the social network promotes liberal news while downplaying conservative, explaining how it picks posts for its “Trending Topics” section, a decision-making process which depends on people and machines at different stages.

Facebook’s VP of Global Operations Justin Osofsky wrote the blog post Thursday in which he explained that in Facebook’s ambition to connect people around major news events and let them together discuss ideas freely, the Trending Topics team reviews themes identified as potentially trending by the website’s algorithms, which measure Facebook user engagement. They confirm the topic is current news, find corroborating news stories, apply a label to the topic, and check if the story is getting attention from a lot of news outlets nationally and/or globally.

Osofsky further explained that because Facebook’s programs deliver posts based on what a user has demonstrated interest in, users will see different Trending Topics.

Once a user clicks to see more about a Trending Topic, however, they are seeing an organization of posts within that topic which are arranged algorithmically — the ones at the top are those that have seem to have the most social engagement because the most people have liked, shared or commented on them.

Justin Osofsky

Justin Osofsky

As far as measures Facebook takes to prevent the type of political bias that was part of the recent accusations, Osofsky wrote that it “does not allow or advise our reviewers to discriminate against sources of any political origin, period,” and that, of course, the algorithm that “surfaces” potentially trending topics has no such bias.

Facebook’s review guidelines include not permitting “the suppression of political perspectives” or “the prioritization of one ideological viewpoint over another,” and that the company regularly reviews the work of the review team members.

In a specific response to the recent allegations, Osofsky wrote, “We take these reports very seriously, and will continue to investigate the allegations. We have found no evidence to date that Trending Topics was successfully manipulated, but will continue the review of all our practices.”