Facebook provided the following announcement to the site:
“In the next few days, we’re continuing to notify more people that if they want to send and receive Facebook messages, they’ll need to download the Messenger app. As we’ve said, our goal is to focus development efforts on making Messenger the best mobile messaging experience possible and avoid the confusion of having separate Facebook mobile messaging experiences. Messenger is used by more than 200 million people every month, and we’ll keep working to make it an even more engaging way to connect with people.”
Facebook already started forcing users in Europe to use Messenger in April; however, after seeing "positive results" its rolling out the change to all users.
You can download Facebook Messenger from the App Store for free.