Grey Stockholm moved their entire agency to Facebook, and a bunch of people applauded while others called it daft. That's all old now that Argentine agency Kamchatka has moved their entire site to twitter.
This site is integrally developed using Twitter's standard functionality. To view its content it's not necessary to have an account. To browse through it you must explore the different tweets following each section's @kchtk_section.
Sure enough, when you have new twitter, the tweet with names at the first account, @kchtk_e works as a menu allowing the right hand side to show the tweets from @kchtk_WhatWeAre, @kchtk_Clients , @kchtk_Cases and so on. It's not using twitter as much as using twitters new layout. Here's a bunch of twitter accounts that will never respond to you, because that would mess with their layout.