They are in the /system partition, which is sized once per Android release and never changed again. If you remounted it as read/write, and deleted them, you would recover no space!
Any application installed in the /data partition can be deleted. That has the space you see and the space you can free. Applications that come pre-installed on the phone and cannot be deleted are in the /system partition. It is a completely separate part of the storage, not accessible to the user for writing and does not count against free space. It is where the OS lives. Even if you do modify this partition, no matter how, you will not get any more free space for your user data
Also, the size of that partition is decided by taking into account that the Facebook app will need to fit there. If you hadn't any preinstalled apps then this size would be able to be smaller and then you would get more space on your device for applications which you actually need to use.