Basically you sum up why having 1 major, huge company that supplies a vast array of website coverage, is a bad idea, aka google adsense/doubleclick which powers so many ad networks. Yes there is facebook/twitter and a few others but google is the one with the crazy huge coverage. Also yes there are other formats but we live in a very digital age where it has potentially better returns for less cost when you use google adsense rather than traditional media. When you are a small company these cost differences are a huge deal.