Facebook’s new TOS require agreement not to use “book” as a trademark — enforceable?

I’ve seen a couple of references recently to Facebook’s revised terms of service, requiring the user to agree not to use Facebook’s trademarks — including, purportedly, the term “book.”[1] (Here’s one such reference, by the Pillsbury law firm.) I wonder who will be the first to challenge that in court — for example, on grounds of failure of a condition precedent?

I think if I were challenging this provision, I might argue that the user’s agreement not to use a particular Facebook trademark implicitly rests on a condition precedent, namely that Facebook in fact has trademark rights in the stated terms. So if Facebook turned out not to have trademark rights in term “book,” then the user’s agreement not to use that term would be automat­ic­ally nullified.

There might be other arguments as well; this is the one that comes to mind right away.

[1] The actual language, though, doesn’t refer to “book” as a trademark. Here’s the language, in section 5.6: “You will not use our copyrights or trademarks (including Facebook, the Facebook and F Logos, FB, Face, Poke, Wall and 32665), or any confusingly similar marks, without our written permission.” Maybe Facebook backed off?