Facebook may at some point charge users for an ad-free experience according to a patent found by Gigaom. The patent application filed in mid-2011 and published at the end of January suggests that the company may allow users to get rid of advertisements for money. Patent application 20130030987 is named "Paid Profile Personalization" and the description reads:
“The user may select one or more social networking objects to replace advertisements or other elements that are normally displayed to visitors of the user’s profile page that are otherwise controlled by the social networking system. In particular embodiments, the user may edit elements on their profile page that are otherwise automatically generated and controlled in design and content by the social networking system. In particular embodiments, the user is billed on a recurring basis for profile personalization.”
The current areas that displays advertisements could be replaced with the users personal photos or something else they choose. The patent also goes on to say that paying users may also have the ability to display special status messages on their pages, rank their friends and hints at users paying money to message people they aren't friends with.
Of course a patent is just a suggestion of the future, it doesn't mean Facebook will ever do this.