eMarketer has a nice article out today about how advertising dollars are still not following online and mobile useage. It's nice but does not really do a good job of quantifying the online marketing opportunity. This chart, from a presentation by Mary Meeker last November does.
That's $50 billion dollars with a "B" as defined by the most renown Internet industy analyst on the planet. Here is Mary's (who left Morgan Stanley to join Kleiner Perkins Caufield & Byers as a partner) entire presentaton "Ten Questions Internet Execs Should Ask and Answer".