“The only clear trend revealed by the analysis was that moderate topics—that is, those that are popular across the political spectrum—are approved and boosted at a higher rate than liberal or conservative topics,” it said in the letter, signed by General Counsel Colin Stretch.

Sen. Thune raised concerns two weeks ago that Facebook’s use of an algorithm to assemble its trending topics section could mislead users. Following today’s letter from Facebook, he said he was satisfied with the social networking company’s actions.

“Facebook has recognized the limitations of efforts to keep information systems fully free from potential bias, which lends credibility to its findings,” he said in a statement.