Mark Zuckerberg’s fortune dropped $3.8 billion Monday as investors weighed reports that a political advertising firm retained information on millions of Facebook Inc. users without their consent.

The world’s biggest social network said Friday that Cambridge Analytica, the advertising-data firm that helped Donald Trump win the U.S. presidency, received user data through an app developer on its social network, violating its policies. The company harvested private information from the Facebook profiles of more than 50 million users without permission, the New York Times reported.