Facebook said on Monday that it had changed some of the procedures for its ‘Trending Topics’ section after a news report alleging it suppressed conservative news prompted a US Congressional demand for more transparency.

The company said an internal probe showed no evidence of political bias in the selection of news stories for Trending Topics, a feature that is separate from the main ‘news feed’ where most Facebook users get their news.

But the world's largest social network said in a blogpost that it was introducing several changes, including elimination of a top-ten list of approved websites, more training and clearer guidelines to help human editors avoid ideological or political  bias, and more robust review procedures. Earlier this month, a former Facebook contractor had accused the company's editors of deliberately suppressing conservative news. The allegations were reported by technology news website Gizmodo, which did not identify the ex-contractor.

The report led Republican Sen. John Thune to write a letter demanding that the company explain how it selects news articles for its Trending Topics list.

Two days after Thune's letter, Facebook published a lengthy blogpost detailing how Trending Topics works even though it rarely discloses such practices. Previously, it had never discussed the inner workings of the feature, which displays topics and news articles in the top right hand corner of the desktop homepage for its more than 1.6 billion users.

Facebook said its investigation showed that conservative and liberal topics were approved as trending topics at nearly identical rates. It said it was unable to substantiate any allegations of politically motivated suppression of particular subjects or sources.

But it did not rule out human error in selecting topics.

"Our investigation could not fully exclude the possibility of isolated improper actions or unintentional bias in the implementation of our guidelines or policies," Colin Stretch, Facebook's General Counsel, wrote in a company blogpost.

Facebook Chief Executive Mark Zuckerberg met last week with more than a dozen conservative politicians and media personalities to discuss issues of trust in the social network.

In his letter, Thune had called on Facebook to respond to the criticism and sought answers by May 24 to several questions about its internal practices.

"Any attempt by a neutral and inclusive social media platform to censor or manipulate political discussion is an abuse of trust and inconsistent with the values of an open internet," Thune said.