With names like "Angry Drivers of Normandy", Facebook groups are the nerve centre of the "yellow vest" protest movement raging across France — and increasingly, a breeding ground for fake news.

When Facebook's CEO Mark Zuckerberg announced in January that the social media giant was going to start prioritising local news, little did he know it would end up feeding the worst crisis of Emmanuel Macron's presidency.

Internet experts say changes to Facebook's algorithms have helped "anger groups" like that in Normandy swell to tens of thousands of members — and last month, they spilled onto the streets.

November 17 marked the start of nationwide road blockades against rising fuel prices, which have since ballooned into a mass movement against rising living costs and Macron in general.

With thousands of posts railing against everyone from the president to a shadowy global financial cabal, the groups reflect the leaderless nature of the yellow vests, who subscribe to a variety of different goals.

What's clear is that the groups have been crucial in mobilising protesters, who mainly hail from small-town and rural France.

"We use Facebook for absolutely everything — informing ourselves, organising ourselves," said Chloe Tissier, a moderator of the "Angry Drivers of Normandy" group, which has more than 50,000 members.

"When a barricade is going up and we don't have enough pallets to set on fire, or food, we put out a post and someone comes to bring them. Doing this by phone would be impossible," she said.

Facebook is also "great because older people are there too", she added.

Pensioners, angry that their allowances are being squeezed, make up a sizeable chunk of the movement.

The power of algorithms

"The yellow vests are not at all a structured movement — there's no spokesman, it's decentralised. So Facebook is ideal for them," said Tristan Mendes France, who teaches digital culture at Paris-Diderot University.

The change in Facebook's algorithms earlier this year lowered the visibility of content published on pages run by large media outlets, he explained.

"It prioritised content being shared by groups, individual profiles, and local information. This change in the algorithm has boosted the emergence of this movement," he said.

This is far from the first time anger fuelled by social media has spilled onto the streets, the 2011 Arab Spring being a prime example.

Sri Lanka temporarily banned Facebook this year after a wave of hate speech posted on the website helped to spark deadly anti-Muslim riots.

Olivier Ertzscheid, a researcher at the University of Nantes, noted that angry content in particular flourishes on Facebook.

"The sentiments which spread most effectively on the platform are those that highlight indignation and anger," he told AFP.

"Facebook offers the technical architecture for spreading information which is perfectly adapted for that."

Rumours fly

A deep vein of anti-elite anger and suspicion of the mainstream media runs through the "yellow vest" Facebook groups — comments not unlike those heard among Donald Trump's voters on the other side of the Atlantic.

"Politicians are fake, the media are fake," reads the "About" section of the "Citizens in Anger" group, which counts 16,000 members.

Yet the "yellow vests" comprise voters of various political stripes, including those who back both the far left and far right in France.

As seen during the US presidential election, though, false rumours have spread like wildfire among yellow vest supporters active on Facebook.

Posts raising alarm that France is about to sign its sovereignty away through a United Nations migration pact have racked up hundreds of thousands of views and shares.

Other viral posts purporting to show police violence against yellow vests use photographs of bloodied protesters taken years ago.

"We try as much as possible to sort through what we publish," said Tissier of the "Angry Drivers of Normandy" group.

But with hundreds of posts an hour, the volunteers moderating such groups struggle to keep up.

Meanwhile, as France braces for fresh violence on Saturday following chaos in Paris last weekend, the varied comments posted on Facebook reflect a movement that is disparate, and therefore unpredictable.

Some see more scenes of torched cars and tear gas in Paris as the only way to get what they want, whatever that may be.

"Do you really think a peaceful movement, even with 100,000 people, can change things? Unfortunately, I don't think so," read one comment on the "Angry France" group.

But many others urged those heading to Paris to refrain from violence, saying it didn't help their cause to be seen as extremist thugs.

"If you act with violence you will be playing the game of the politicians," read another post on the group, which has more than 300,000 members.

"Show them that we're human."