Facebook activity among Britons has crashed by more than one third over the past 12 months, according to the analytics firm Mixpanel, first reported by The Daily Telegraph.
Since June 2018, several months after news of the Cambridge Analytica scandal was revealed, activity on Facebook's mobile app in the UK dropped 38% through June 2019.
User clicks on web links or adverts inside the Facebook app declined in seven of the last 12 months, with an average monthly drop of 2.6%.
The alternative data outlines an entirely different story from Mark Zuckerberg, the chief executive of Facebook, who recently said Europe's monthly active users continued to rise.
Investors have traditionally viewed the total number of users as a reliable metric of the social media company's health, but with millions of fake accounts, many have turned to alternative data that shows a mass exodus of users started in the UK.
Last July, $120 billion was wiped off Facebook's market value after it reported an unexpected drop in European users and shifted guidance about future growth lower.
Zuckerberg said last October that users in the US have plateaued and that future growth for the company would come from emerging markets.
Mixpanel's data, also, estimates how users open web pages or services on Facebook, provided another form of alternative data that serves as a proxy of user activity.
Matti Littunen, a social media expert at Enders Analysis, questioned the alternative data - didn't believe the figures represented an accurate view of Facebook users' activity across the UK, due to his belief that Facebook's usage data showed an uptick. Instead, Mixpanel's data could reflect changes in advertising tactics, he said.
Littunen said if usage does begin to fall - advertising prices will start to rise as firms compete for smaller audiences, leading advertisers to shift ad money elsewhere.
"Facebook has reached a very high level of user saturation in the core markets like the US and the UK, meaning that they have little margin for error before engagement drops from the peak," he said.
"If Facebook usage were to drop by a third, Instagram would have to double in size to make up for it.
"No messaging app has supported a multi-billion dollar advertising business so far, so WhatsApp and Messenger would not be able to make up for a major shortfall."
In a separate report, advertising research firm eMarketer said in May that users had spent an average of three minutes less on Facebook in 2018 than they did in 2017.
"On top of that, Facebook has continued to lose younger users, who are spreading their time and attention across other social platforms and digital activities," eMarketer said.
Another Mixpanel report shows likes, shares, and posts have fallen 20% since April 2018.
The decline in Facebook activity by Brits coincided with privacy and hate speech scandals throughout 2018. In September, the company disclosed that a security breach exposed 50 million accounts - further deterring users from using the social media platform.