Facebook's Messenger app now has 1.2 billion monthly active users since first launching in August 2011.
Messenger was spun-off from the main Facebook app to allow users to focus on exclusively messaging their contacts.
Some users complained that it requires them to download two apps, though the Facebook app does not need to be installed to use Messenger.
"I keep on hearing powerful stories about how our product is becoming a more important part of your daily lives," wrote Facebook's Head of Messenger David Marcus on his announcement post for the milestone.
"So from all of us here at Messenger, a heartfelt thank you to all of you for giving us a chance to build something good and more meaningful for you."
The figure is especially good news for HTML5 developers looking to get their games on Facebook's Instant Games platform.
First launched in November 2016, Instant Games allows users to play small games with their friends directly from the app.