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.