Someone gaining access to your messages is a nightmare. With Lock For Facebook, you can safeguard your chats from prying eyes and hands. It protects the following apps: Facebook, Facebook Lite, and Messenger. 

You can create a numeric password of up to eight digits (minimum of four characters). Further, Lock For Facebook lets you change your password in case your previous one is too weak or common. This locker can be disabled at any time. In addition, you have the option to share or rate the application. This tool doesn't take up too much of your storage, too, making it lightweight.