This is the most dangerous step. IPA archives on the internet are famous for containing malware, adware, or spyware.
If you have previously downloaded Facebook on your Apple ID (even on a different, newer device), you can install a working version on your older device: Open the on your iOS 9.3.5 device. Tap on the Purchased tab at the bottom. Search for Facebook and tap the cloud icon . facebook ipa for ios 9.3.5
: On your iOS 9 device, open the App Store , tap the Purchased tab, and search for Facebook. This is the most dangerous step
If prompted with "Download an older version of this app?", select 2. Side-Loading via AltStore or Sideloadly Tap on the Purchased tab at the bottom
If you are developing a custom feature within an IPA, you can sideload it using modern tools adapted for legacy devices. Sideloadly
This is the biggest deal-breaker. Facebook frequently updates its server-side APIs (the code that lets the app talk to Facebook's servers).
Since the legacy Facebook app often suffers from login errors or crashing on iOS 9, the most stable way to use Facebook is via Safari. Navigate to m.facebook.com Share icon and select "Add to Home Screen" to create a shortcut that looks like an app.