Facebook says that it has added the feature for those who "share their phones and tablets with members of their family and friends." And even though more than one person will be able to avail themselves of the Facebook Messenger app on one particular phone or tablet, messages stay private.
A password is required the very first time a multiple account is opened on the app. After that, the account holder can decide whether or not a password is needed.
The beauty of this new feature is that your messages stay completely private. Anyone else who shares the device will just see the number of notifications of messages that you have, but they will never be able to see the content of your messages."-Facebook
The update to the Android version of Facebook Messenger could hit your device at anytime. You can try to manually update the app by opening the Google Play Store, going to My apps and games, and checking your pending updates.
source: Facebook via AndroidCentral