to an Apple ID that has downloaded Facebook in the past on a newer device. on your iOS 9.3.5 device. and search for Facebook. Tap the cloud icon; a prompt will ask if you want to "Download an older version of this app" Community Perspective
Apple allows users to download the last compatible version of an app if they previously owned it. However, if you have a new Apple ID or have never downloaded Facebook before, the App Store blocks you. The only workaround is manually installing an IPA (iOS App Store Package) file. facebook ipa for ios 9.3.5
This is the most traditional method for installing IPA files without jailbreaking. to an Apple ID that has downloaded Facebook