Yeah, I don't think Facebook is crutching the trademark argument here at all: it is just thrown into the list of complaints to make them longer ;P. I additionally think that "F.B. Purity" could probably be defended (if not on "nominative use", then on how Facebook themselves largely prefers "f" to "FB" in their branding in addition to not capitalizing the B in general at all)... what I dislike is having that argument "they don't own the letters FB" thrown in there, as that isn't how trademark claims work. :(