Since its release in 2014, the Facebook Groups app has helped people access and share more easily with all of their groups; however all good things must come an end.
Recently, Facebook announced that they will be discontinuing the standalone Groups app for iOS and Android after September 1, 2017. The reason behind this is that Facebook is going to focus on groups in the main Facebook app and on facebook.com. But fret not. All of your groups from the Groups app will be available in the main Facebook app, where you can continue to connect with all your communities.
Source: Facebook
