Facebook is trying to expand its trademark rights over the word "book" by adding the claim to a newly revised version of its "Statement of Rights and Responsibilities," the agreement all users implicitly consent to by using or accessing Facebook.
You may recall that Facebook has launched multiple lawsuits against websites incorporating the word "book" into their names. Facebook, as far as we can tell, doesn't have a registered trademark on "book." But trademark rights can be asserted based on use of a term, even if the trademark isn't registered, and adding the claim to Facebook's user agreement could boost the company's standing in future lawsuits filed against sites that use the word.
"Unregistered marks are quite common in the US," University of Minnesota Law Professor William McGeveran told Ars. "Rights arise from use, not registration (though registration does give you some other advantages). That's how Facebook can try to claim 'book'." If you see a ™ next to a name, that indicates an unregistered, claimed trademark, whereas an R in a circle signifies a registered one, McGeveran notes.