Since the Google Play Store likely won't let you download compatible versions automatically, you will need to sideload the app using an APK file.
Prerequisite: Before you begin, you must enable installation from unknown sources on your phone.
Where to find the file: You can search reputable APK repositories online (such as APKMirror or Uptodown) for "Facebook for Android." You will need to look for version numbers around 1.5 to 1.9.
Installation Steps:
Since you cannot use the Play Store, you must install an APK file manually.