People who are familiar with how trademarks work can still be dubious of Facebook's attempt to register BOOK. Is there a context in which I would accept it as distinctive? Yes, of course: the name of a social networking service. Is Facebook applying under such narrow terms? Not as far as I can tell. And do I trust Facebook to be tasteful and show restraint in defending this trademark, if it's granted? Not in a million years.
P.S. This is almost beside the point, but remember when Monster Cable sued a bunch of small businesses—most famously a mom-and-pop mini-golf course—over trademark infringement?[0] At the time, there was no shortage of pedants chiming in to say that Monster didn't _need_ to be such a bully, that it wasn't an issue of the trademark system per se. As if that were some consolation!