"we were unable to determine if left-wing news topics or sources were similarly suppressed"
I see what you're getting at, but I highly doubt liberal news was systematically suppressed. Facebook is a notoriously far-left company, located in a region of the US that is already far-left.
Silicon Valley has a fairly substantial right-libertarian and just-plain libertarian segment, which, in the usual way multidimensional politics are smushed into the US two-party, one-axis system, fall out as "conservative", (and quite a lot of really hard to classify on a liberal-conservative axis types that are extremely socially liberal, but economically center-to-right-libertarian (that is, outside of culture-war kind of issues, they tend to agree more with the center-to-right types on the function of government, as well as leaning libertarian on its preferred degree and means of involvement in fulfilling that function); these don't fit well as either "liberal" or "conservative" in the classical sense, and often aren't very "moderate" though they might get classified that way as a result of being very liberal on some salient issues and very "conservative" (as that works out in the US political spectrum) on others.
If you read the article, its quite possible that it was also liberal news, depending on individual curators. Given other FB interventions I've seen, I'd be surprised if, like everything else at FB, the news moderation wasn't just stunningly inconsistent with rampant suppression of left, right, and center according to individual and essentially unaccountable whim.
FB is tolerably usable for basic circle-of-immediate-friends social functions without excessive interference. As far as public communication, news, and advocacy, well, its there, and there's enough people that come to it for that that it can't be ignored, but its fairly horrible for it, not so much from a systematic one-sided distortion but more absolute caprice.