CMS = content management system... they're online, otherwise they wouldn't be able to serve content to clients that need to download it.
Hiding the login page won't stop hackers. Most don't go to Google and type "login page" to find a site to break into. Besides, small sites wouldn't appear in the first several hundred results. But if you want to block search spiders, use a robots.txt file. But this will not stop a malicious spider, because it's their choice to adhere to it or not.
Facebook and MySpace use their own code, or "content management system" - not a script that you can install...
I'm not totally sure what you're trying to figure out... explain a little more maybe?