Why did my Facebook messages disappear?

Facebook has begun phasing out mobile messaging on its main app. Users who want to message their Facebook contacts on their iOS device will need to download the separate Facebook Messenger app. The app is free for all devices running iOS 7.0 or later. It allows users to message and share photos / videos with other Facebook members. The oldest version of Facebook Messenger should be compatible with iOS 4.2.1 and the iPhone 3G.