Facebook has reactivated Messaging in App. Messenger no Longer Forced.
Facebook has stopped forcing users to download the Facebook Messenger app and has re-enabled in app messaging. The nag screen still exists, but now you may bypass it.. Good news for users such as myself with low storage capacity and no room for an unwanted app.
Current Facebook version: 188.8.131.52.12
If there's any problem, try clearing your cache to reactivate the "remind me later" button on the nag screen.