Facebook said that there are "rigorous guidelines in place" to maintain neutrality and that these guidelines do not prohibit any news outlet from appearing in trending topics.
"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," a spokesperson for Facebook said.
A post on Facebook's help center said that the "trending" section of the site "shows you topics that have recently become popular on Facebook." It lists "engagement, timeliness, Pages you've liked and your location" as some of the factors that determine what trends show up for each Facebook user.