Facebook tests Face ID, Touch ID lock for Messenger

Similar to what it already implements on WhatsApp, Facebook is testing the capability to lock the Messenger app on iOS using Face ID or Touch ID authentication. You can opt to require authentication every time the app is opened or after some time without use (ranging from one minute to an hour). Facebook is introducing this biometric authentication feature to help secure users' private messages. It's currently available for a small number of Facebook Messenger users, but the company plans to expand its use in the future. It won't say when, so if you don't have it yet, you're going to have to wait. 

Source: 9to5Mac