A webpage which claimed the pontiff had backed Donald Trump was widely shared and appeared on millions of people's news feeds despite it being false.
Francis had only made one comment on the US election and that was to say that building walls was not Christian. This was in reference to Trump's plan for a border wall between America and Mexico.
The fake news story, which was not taken down or flagged up as false by either Google or Facebook, even contained a hoax quote from the pope.
"I have been hesitant to offer any kind of support for either candidate in the US presidential election but I now feel that to not voice my concern would be a dereliction of my duty as the Holy See," the pontiff was claimed to have said.
Google and Facebook have announced plans to fight the spread of fake news by placing tighter restrictions on how such sites make money from advertising.
The announcement comes following criticism of social media over the role it played in the US presidential election, and the alleged spread of false information that may have influenced some voters.
Facebook founder Mark Zuckerberg has denied claims that the social network played any role in influencing the election result, claiming that "99%" of the content on Facebook was authentic.
Matt Chorley - the editor of The Times newspaper's Red Box email service said: "The famous one that did the rounds was that Pope Francis had backed Donald Trump. Things linger and the more they're liked and shared the more they linger on people's timelines.
"People think that Facebook is a big trusted brand and they start to trust the things that appear on there."
More Catholics voted for Donald Trump that did for Hillary Clinton in the election.
