Maybe we should :) Although I think hackerbook.com would be a better name for some sort of collaborative book project.

I get what you're saying, but if you look at it from Facebook's point of view if they don't defend their trademark they'll lose it. If there are looks of [something]book.com social networks then [something]book.com just becomes a generic term for a social network. Facebook then loses the right to claim any sort of exclusivity at all on their trademark (I think that's how it works anyway, IANAL obviously)