Facebook Messenger for Android now supports using multiple accounts on the same device, allowing users to easily sign into and switch between accounts on the fly. The feature, which has been in testing for some time, is now accessible to all users through the Accounts section of the Settings menu.
As Facebook notes in its announcement post, this feature is really geared towards those that share their tablet or phone with other family members on a regular basis. And if you're worried about keeping messages private, it sounds like you should be in the clear. The first time you switch from one account to another, a password will be required, and you can choose whether to always require one or not. When new message notifications come in for another account, the contents will be hidden until you switch to that account.
The addition of multiple account support for Facebook Messenger comes just a week after it made its official appearance on Facebook-owned Instagram.