In his open letter and report to Republican Senator and Commerce Committee chairman John Thune, Facebook's General Counsel Colin Stretch went straight for the numbers, which it pulled from 90 days of data:
Our data analysis indicated that conservative and liberal topics are approved as trending topics at virtually identical rates. We were also unable to substantiate any of the specific allegations of politically-motivated suppression of particular subjects or sources. In fact, we confirmed that most of the subjects mentioned in media reports were included as trending topics on multiple occasions.
Although Facebook believes it acted appropriately in the past, Stretch's statement does mention that the company can't rule out "the possibility of isolated improper actions or unintentional bias" in the way its Trending Topics policies were carried out. Facebook also noted that, prior to July 2015, certain topics may have been excluded from the section if none of the 10 major news outlets had written stories on the about them. In order to prevent unintentional bias (or just more accusations of it) in the future, the social network behemoth has updated and clarified its internal policies as well as mandated additional refresher training for topic reviewers.
For his part, Senator Thune responded, saying his committee found the report acceptable and that Facebook's openness in the face of a congressional inquiry was refreshing. "While the committee remains open to new information on this matter," Thune wrote, "transparency – not regulation – remains the goal." So, to keep up the appearance of transparency for now, Facebook has helpfully updated the relevant Help Center section.