Finding and installing a functional Facebook app on iOS 9.3.5 can be tricky since the official App Store version typically requires a much newer iOS.
"Login Failed" Error: If the app installs but won't let you log in, try disabling Two-Factor Authentication (2FA) on your Facebook account temporarily. Legacy apps often don't know how to trigger the 2FA code popup. facebook+ipa+for+ios+935+best
Because the app often fails to load content on legacy iOS, using a web browser is the most reliable "best" way to use Facebook on iOS 9.3.5. Finding and installing a functional Facebook app on iOS 9