Many IPA files found on third-party sites may be outdated or insecure. Always prefer downloading through the official App Store "Purchased" method or using the web version to protect your account data. that still work on iOS 9.3.5? Old iPad iOS compatibility workaround? - Facebook
Switch back to your iOS 9.3.5 device, open the App Store, and go to . facebook ipa for ios 9.3.5