Rooted means gaining root/superuser access to the phone, which also allows access to system files. Facebook is considered a system app and thus, only available to be removed if you have that access level. It sounds like you are completely stock, so the only option is to not use it. However, if its an icon on your screen, you can remove that from the screen (same with any widgets or facebook apps other than default). Also, I know LauncherPro (home screen alternative) offers the option of hiding apps, if you really can't stand seeing it in your app drawer. I think other home screen replacement apps do as well, though am not certain.
Also, if you're not going to use it, feel free to disable any notifications from it, logout from the app (if possible, I don't use facebook), and go into settings and delete data and cache for it. That'll at least reduce its impact on your phone.
