Gizmodo's story today said journalists hired by Facebook routinely kept stories from right-leaning news sources out of its Trending news section. However, some ex-Facebook News employees, reacting to the story, said quite the opposite was true–that unbiased news judgement was carefully applied, and that right-leaning news stories were not excluded unfairly, at least not routinely or systematically.
One ex-Trends curator said stories were sometimes omitted from the Trends list if the news source wasn't a known name, and if the story hadn't also been picked up by at least three mainstream media outlets. The source said curators would, however, sometimes exclude popular-but-trivial stories (celebrity gossip, for example) so that hard news stories could be included in Trends.
So there may be two sides to the story Gizmodo's Michael Nunez tells, and it's a bit of a he-said/she-said thing now anyway. Still, Facebook reacted aggressively to the story in a Monday afternoon statement:
There are rigorous guidelines in place for the review team to ensure consistency and neutrality. 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. These guidelines do not prohibit any news outlet from appearing in Trending Topics.