More than that, it appears Facebook wouldn't curate a story with conservative origin (Breitbart, for example) unless it was picked up by The New York Times or BBC first. While Facebook's company line is that it "takes allegations of bias very seriously" in light of the Gizmodo report, claiming "rigorous guidelines" to ensure consistency and neutrality and that those guidelines don't "permit the suppression of political perspectives," the sources for these allegations were contract workers -- not full-on employees themselves. These contractractors worked for Facebook from the middle of 2014 until December 2015.
What appears in the Trending News module isn't exclusively determined by an algorithm of what its users are actively sharing, it's curated much like how an editorial newsroom operates. One of Gizmodo's sources -- who leans politically conservative -- says that what would populate the list was largely determined by who was working at the time. If that person happened to not subscribe to conservative points of view, a story would be blacklisted. More than that, if a particular story is trending on Twitter but not Facebook? It's "injected" into the Trending News section. Specific instances of that include the Black Lives Matter conversation or the ongoing conflict in Syria.
This isn't the first time Facebook has come under fire for this type of thing. In 2014 the company admitted that it controversially, and experimentally, altered the News Feed to measure your emotional responses.
Update: Tom Stocky, Facebook's Vice President of Search, has posted the following statement refuting the Gizmodo report: