If you're wondering whether you can view private Facebook content or bypass Facebook's security and privacy restrictions using Greasemonkey, then the answer is "I highly doubt it".
Why would it be impossible to use Greasemonkey to view private Facebook content?
Greasemonkey is a browser extension created for Mozilla Firefox that uses Javascript, XUL and CSS to make changes to what a user sees in the browser.
All the changes that Greasemonkey makes, no matter how sophisticated, happen on the client side and only affect the person using the browser on which it is installed.
Facebook handles all security and access requests through it's internal system, so this wouldn't be affected by changes to the way the site renders etc...
To view private profiles on Facebook through some sort of hack, it's likely you would need to use advanced PHP to exploit a specific weakness in the public API for example.
You can always try using Greasemonkey though... I could always be wrong!
It's technically not a hack at all, just customization to how your browser interprets the code Facebook is already giving you access to.