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Facebook's algorithm puts hoax story about 9/11 on Trending Topics

The top link for Facebook's 1.7 billion users was a hoax, claiming that the World Trade Center was destroyed by bombs.

Facebook got hoodwinked again.

On Friday, Facebook's algorithm for selecting trending stories placed "9/11" at the top of its list, linking to an article by The Daily Star, which claimed the Twins Towers fell because of bombs planted within the buildings, instead of a terrorist attack.

It's the second time in two weeks Facebook's algorithm posted a fake news story on the top of its Trending Topics. On August 27, it posted a hoax story about Fox News anchor Megyn Kelly -- just a day after the social network switched from its human curation team.

With more than a billion active monthly users, Facebook's Trending Topics section is prime real estate for views, whether or not the stories are true. The company ran into hot water when it pulled the iconic "Napalm Girl" photo from the social network before changing its mind about its rules. Fake stories on the sidebar can lead to mass misinformation among its users.

The hoax post was promptly removed once Facebook was alerted to the post.

This was the article linked to Meghan McCain at about 9 a.m. PT on Friday.

Screenshot by Alfred Ng/CNET
screen-shot-2016-09-09-at-4-09-29-pm.png

By 2 p.m. PT, the link tied to Meghan McCain's trending topic had shifted to a Yahoo article.

Screenshot by Alfred Ng/CNET

"We're aware a hoax article showed up there and as a temporary step to resolving this we've removed the topic," a Facebook spokesman said in a statement.

9/11, like other trending topics, had landed on the list organically, from people anticipating the 15th anniversary of the terrorist attack, but the link associated with the topic had been plagued by the hoax article.

The articles are placed through an algorithm based on relevancy and time posted, according to a person familiar with the matter. It replaces the links after an undetermined amount of time, without any human supervision or quality control.

At about 9 a.m. PT on Friday, Meghan McCain, the daughter of Arizona senator John McCain had been a trending topic linked with an article from Prntly -- which the Washington Post called a "pro-Trump fake news website" -- calling her "Miss Piggie," the Muppet character.

By 2 p.m., the leading link had changed to a Yahoo article about McCain criticizing the 2016 presidential election.

When Facebook switched to its algorithm on August 26, the company said it would still have people involved in the process to make sure the links posted remained "high-quality."

Facebook declined to comment on how its quality control team missed the algorithm's hoax links before it was posted in the coveted trending section.