Facebook announced today that it's having over 500 million people on Messenger each month, more than doubling its user base since last April. The reason of the huge expansion of the app is obvious: Over the summer, Facebook removed the ability to use Messenger inside of its primary mobile app. Instead, everyone who receives a message within the Facebook app is now prompted to download and move over to Messenger in order to see and reply to messages.

Facebook says that the move was necessary in order to provide a better experience in both messaging and the News Feed.

Facebook still has a lot of work to do on Messenger, as the majotity of the social network's user is obviously not using the Messenger app yet. In addition, the Messenger app is still falling short of WhatsApp, which has over 600 million monthly users.