Communities like r/LegacyJailbreak on Reddit are excellent resources for finding stable versions of Facebook that still communicate with modern servers.
Use a tool like Sideloadly or AltServer-Patcher on your computer. Plug in your device, drag the IPA into the tool, and enter your Apple ID to sign it.
tweak allows the OS to ignore signature checks, enabling a permanent installation of any Facebook IPA without seven-day re-signing limits. Filza File Manager
iOS 9.3.5 is nearly a decade old and lacks modern security patches, leaving it open to kernel-level attacks IPA Warnings Downloading