This screen has been showing for quite a while whenever you access messages via the mobile app. It prompts to install Facebook Messenger. In the future it seems the option to cancel and continue to the messages will no longer be available, with a few exceptions. Those exceptions are low powered Android devices with limited memory, like the Galaxy Ace.
Windows Phone and tablet users will keep the functionality in the app and people using the new Paper news app will also retain messages. Although this may change in the future, especially in the case of Windows Phone and tablets, if a Messenger app becomes available.
This has pros and cons, on the plus side messaging on Messenger is apparently a lot easier, don’t use it so can’t confirm, and it should hopefully slim down the Facebook app and allow users to make full use of the features like calling in Messenger. However it does mean having to find space for another icon on your homescreen and switching between apps is always a pain.
Source: The Verge
Received this email from Facebook this morning, the split is definitely happening.
We wanted to let you know that messages are moving out of the Facebook app because our Messenger app is much faster. You’ll receive new messages instantly and be able to reply straight away. In fact, we’ve found that people reply about 20% faster on Messenger than on Facebook. So as your friends start using Messenger, you’ll probably notice them replying to you sooner.
We’ll keep giving you links to install Messenger, and soon you’ll see a reminder notice where you’d normally see your messages. At that point, you’ll need to install Messenger or go to the Facebook website to view and send messages.
We’re excited for you to try out Messenger. We’ll be adding new features and making improvements, so please tell us what you think.
The Messenger Team