In its revised "Statement of Rights and Responsibilities," Facebook claims the word "book" as a trademark. This may seem both pedantic and silly, but Facebook has always been protective of its namesake.
Jon Brodkin of Ars Technica clears things up in his article on the issue:
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.
Brodkin deserves praise for his well-researched and thoughtful take on the issue, but this nuanced view of what's going on is obviously lost in the din of Twitter. The expected ridicule is flying:
(Via ars technica)