Facebook has been monitoring the mobile phone habits of Facebook users via Onavo Protect’s free VPN even when they’re not using the social media platform.

The Independent reports that Facebook has been using data gathered from Onavo Protect, a free VPN app that claims to help “keep you and your data safe when you go online,” which was created by a company purchased by Facebook in 2013. Facebook uses data gathered from Onavo Protect to learn which apps Facebook users open, how frequently they use them and even how long they use them for. This information is then used to shape Facebook’s product roadmap, helping the company to decide where they should focus their development efforts in the future. Reportedly, the data gained from the VPN firm was a key influence in Facebook buying Whatsapp and copying Snapchat’s stories feature.

The information was reportedly so precise that Facebook was able to keep track of how many Snapchat posts Facebook user sent each day. Facebook allegedly uses the information to determine what apps they’re competing with for screen time in order to learn how to improve their own products and what areas they should focus on. Onavo Protect is available on both Android and iPhone and has been downloaded by around 24 million users. The Independent reports that when anyone with Onavo Protect on their phone opens a web page or app, that information is redirected back to Facebook servers.

This information is then logged to a database which is analyzed by Facebook product engineers. Apple and Google reportedly use similar methods to collect data on their users but it’s unknown if this information is used to improve these companies products. Onavo does state that it “analyzes information about your mobile data and app use” and may share the data with “affiliates”, but it’s quite likely that many users of the app are unaware that their mobile usage data is being transferred back to Facebook servers.

The data gained by Facebook from Onavo Protect allowed the company to measure how devastating the inclusion of a Stories feature on Instagram was to Snapchat which had previously pioneered the feature. Snapchat released details on the effect that Instagram Stories had on their own app long after Facebook had already measured the success of their new feature using information gained from Onavo Protect.