I can only speak for people I know, but as far as I can tell the phone book is being replaced by (a) cell phones, which until the iPhone mostly had horrible UI

I can only speak for people I know, but as far as I can tell the phone book is being replaced by (a) cell phones, which until the iPhone mostly had horrible UI design and (b) Google, which goes barely beyond Craigslist in the visual design department. I don't know anyone who uses YellowPages.com. (According to Alexa its reach is 0.1%.)