Did you try clearing the Play Store app's cache? Go to Settings > Apps > All apps > Google Play Store > Storage > Clear cache (This may be a little bit different depending on your device), and then try to download Facebook again. If this didn't work, also try clearing the Store's data. Again, go to Settings > Apps > All apps > Google Play Store > Storage, but this time press Clear data. Open Google Play Store again (it may ask you to sign in or accept terms of service) and try to download it again. (Note: your Play Store settings will be set to default again).