Users of the separate Messenger app have more than doubled from 200 million in April, the company said.
Facebook had faced a backlash after it announced it would be forcing users to download the app to send private messages to friends over the main Facebook service.
Users started being pushed to download the app - launched in 2011 - in July.
"Messaging is an important part of how people stay connected and since Messenger launched in 2011 we've been passionate about giving people a faster and more expressive way to communicate," said Facebook's director of product management Peter Martinazzi in a blog on Thursday.Facing criticism
In April, Facebook had told users that they would need to download a separate app to allow them to send videos, make free calls and chat with friends.
That led to the iOS App Store review score for the recent version of the Messenger app to fall to one and half stars.
"Messenger was the first of our standalone apps, and unlike our core Facebook apps, it focused on one use case - messaging," Mr Martinazzi said.
"We've also continued to improve speed and reliability. Updates to Messenger ship every two weeks so it continues to evolve and improve," he added, showing that Facebook plans to stick with the app despite the negative feedback.
The mobile messaging app had more than 600 million monthly users at the time of the acquisition.