When 99% of your friends use Facebook, it becomes very hard to avoid it [0]. If you want to advertise your new book among them, what would you do, for example?
Through one of the many other sites those friends probably use. Facebook probably isn't the only thing on the internet you and your circle of friends have in common.
Yes, but the other sites/services they use are also all pretty much similar walled gardens. It's becoming rarer and rarer these days for ordinary web users to have their own domain and site, or meet upon a completely unrestricted communications medium, like the Usenet of old.