No, it's because because "facebook" has never been used as a common noun to describe social networks - as in, "I'll go log onto the facebook," referring to MySpace or LiveJournal or something. This conversation is about businesses which are named after the field they are in. (The equivalent is if Facebook were named "Social Network.")

Also, I doubt most people have really heard of the concept of a physical facebook. From my understanding, it's limited to Harvard and maybe other schools around there.

They completely dominated the space that the traditional facebook was in though, that's what they started out as. Obviously they've grown to encompass a much wider space since then.