Facebook said its "news curators" didn't suppress conservative news outlets in its trending news module, even though they have the capability to do so.
On Monday, Gizmodo cited anonymous sources who claimed to be former Facebook contractors and said they routinely weeded out news from conservative sites while moderating the "Trending News" section.
Facebook denied the accusation.
Tom Stocky, who is in charge of Facebook's trending topics team, said that after an algorithm picks out popular topics for the module, humans audit the results. During that review process, bias can easily seep into what's considered top news.
But the top news module is small and found on mobile devices only after a search. Some say it would be of more concern if Facebook used personal bias to curate users' main news feeds, which could happen.
In 2014, Facebook admitted to tweaking the news feed, or at least to letting researchers experiment with the feed.
The team filtered whether users saw more positive or negative posts for a week, then tracked whether the users' own posts became more positive or negative in response.
The older manipulation of the news feed and the new allegation about the top news module serve as reminders that social media sites can ultimately control what you see on their sites.
Every time a user logs in to Facebook, he or she could potentially see about 1,500 posts on a news feed, but Facebook's algorithm narrows that number down to a less overwhelming 300 posts or fewer.