Find out more about the change after the break.
Clicking "Not Now" adds the request to a separate menu of "Hidden Requests." Sending a person's request to this group seems a little like Facebook Purgatory — because it is. You can revisit this list later and approve them at a later date should you change your mind. Or, you can choose the "Don't know this person?" option, and Facebook will block them from being able to request your Facebook friendship in the future, preventing annoying and spam-like requests.