This is the most critical failure point. While an older APK can be installed, Facebook’s backend servers evolve. APIs change. A version that is too old will fail to log in, crashing with an "Unknown Error" or "Server Update Required" message. The demand for a specific recent legacy version (like 4120005) suggests a version that is old enough to be light, but new enough to successfully handshake with current authentication servers.
: Lists a long history of older APK files, including sizes and version numbers. AndroidAPKs facebook 4120005 apk older versions for and top
: Specializes in legacy software, hosting very old versions of the Facebook app (dating back several years) for archival purposes. Safety Considerations This is the most critical failure point