The point is that facebook didn't invent face, didn't invent book, and didn't invent wall (not even in the ACTUAL meaning they are used in facebook itself).
If I want to create a website with social tools that for some reason involves wall or book I don't see why I have to risk to be suit based on a user agreement (that usually nobody reads).
Say that I want to open a social network for old book lovers. I can't call you the old book or the ancient book or whatever you can think of just because facebook thinks they own the word book?
Or what if I want to use the metaphor of the berlin wall to open a thematic social network for human right causes and maybe also use the idea that you can place notes on this wall?
Do you think that in that case am I actually infringing the Facebook trademark?
Yet I'm sure they could try to sue me if I'm a Facebook user and agreed to their fluff.