META Meta Platforms Inc

Facebook Inc. told a U.S. senator that an internal investigation had found "no evidence of systematic political bias" against conservatives in its trending topics tool, but that it will revamp how the feature works to minimize the potential effects of individual biases.

In a 12-page response to Sen. John Thune, (R., S.D.), Facebook said a review found that conservative and liberal topics were approved for the trending feature at "virtually identical rates." The two most frequent topics since early 2015, Facebook said, are Republican presidential candidate Donald Trump and the subject "#GOPDebate."

Still, Facebook said it couldn't rule out the possibility of "isolated improper actions or unintentional bias" by workers in how trending topics are selected. The company said it is making several changes intended to "minimize risks where human judgment is involved."

Among other things, Facebook said the curators who assemble and approve topics will no longer rely on external websites and news outlets to assess the importance of potential topics. Facebook said it its adding more oversight to the trending topics team, but didn't offer specifics.

The letter is Facebook's latest attempt to contain the fallout from a May 9 report published by tech blog Gizmodo that said curators of Facebook's "trending topics" feature suppressed news about conservative events and from conservative sources. Facebook denied bias but revealed that curators had more oversight over what appears in this feed than it previously disclosed.

The Gizmodo report prompted Mr. Thune to send a letter to Facebook Chief Executive Mark Zuckerberg asking how trending topics are selected. On May 18, Facebook general counsel Colin Stretch met Mr. Thune, the chairman of the Senate Commerce Committee. That same day 16 top conservatives met Mr. Zuckerberg at Facebook's headquarters in Menlo Park, Calif.

"The seriousness with which Facebook has treated these allegations and its desire to serve as an open platform for all viewpoints is evident and encouraging and I look forward to the company's actions meeting its public rhetoric," Mr. Thune said in a statement Monday, in response to Facebook's letter.

One conservative who attended the meeting with Mr. Zuckerberg last week applauded Facebook's moves.

"I think this is a good step," said Brent Bozell, president of conservative media watchdog the Media Research Center, in a statement. "Facebook was relying on a preponderance of liberal and leftist 'news' organs. By not relying on any specific news outlets, Facebook returns to its neutral roots."

To draft its response, Facebook said it analyzed all trending topics reviewed by its curators during the past 90 days. The company looked at more than 3,000 decisions to see if conservative or liberal topics were disproportionately boosted or suppressed by reviewers. It interviewed current and former reviewers.

Facebook said the review uncovered no bias against conservative news sources or topics. It said it reviewed more than 600 decisions involving specific topics that the Gizmodo article suggested had been suppressed and found that topics related to Republican Texas Sen. Ted Cruz, conservative commentator Glenn Beck, former Navy SEAL Chris Kyle and former Internal Revenue Service official Lois Lerner had all been accepted.

On another topic raised by the Gizmodo article, Facebook said the Conservative Political Action Conference had been accepted as a topic on each of its three days in 2015 and two days in 2016. The day the conference wasn't included, Facebook said, was the day after the 2016 Super Tuesday primaries, when at least 15 other topics related to the Republican presidential campaign were accepted. There was no way to independently verify Facebook's conclusions.

Facebook said it uses as computer program to identify potential trends from the vast amount of content posted on the social network each day. More than 1.6 billion people world-wide, including 222 million in the U.S. and Canada, check Facebook at least once a month.

Facebook said it uses human editors "to bridge the gap between what an algorithm can do today and what we hope it will be able to do in the future."

Prior to Monday's changes, curators on the team relied on a variety of external sources to identify and verify trending topics. They assigned an "importance level" to news stories based on how they were covered by 10 major news outlets, including The Wall Street Journal, Fox News, the New York Times and the Huffington Post. They also used a list of 1,000 media publications known internally as the "Media 1K" to help write topic descriptions.

Under the new rules, the team will no longer rely on external websites and news outlets; they will have to rely on Facebook's internal data to vet and select its "trending topics." They will only be able to use chatter on Facebook to surface trending topics and all topics will now be assigned the same level of importance, a Facebook spokeswoman said. Reviewers will use Facebook search results to help them write descriptions.

Write to Deepa Seetharaman at Deepa.Seetharaman@wsj.com

(END) Dow Jones Newswires

May 23, 2016 21:35 ET (01:35 GMT)

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