Users choosing the “Protect” option in the Facebook iOS app are being sent to download a VPN app that also allows Facebook to track their phone use, TechCrunch reports. Facebook acquired Onavo Protect and its VPN technology in 2013, but iOS users in the U.S. are just starting to see the redirect link in the main Facebook app that leads to the Onavo Protect – VPN Security app in the App Store. The app promises to safeguard private information and provide warnings about malicious websites, but it also gives Facebook the ability to monitor activity across all the apps on a user’s phone. Facebook reportedly uses those insights to spot new apps that are surging with key demographics and decide which apps to buy before they blow up or which features to steal from other successful apps, like when the company undercut Snapchat by adding similar features to Instagram. The Onavo Protect app is currently installed on more than 33 million devices, and it’s unclear how many of those users know just how much of their data is being sent back to Facebook while it’s in use.