(click image to enlarge)
This is one of the worst data visualizations I've seen. Problem is it looks pretty, so people send it around, but it's not very informational. Nor does it allow easy comparison of the data. First, it's not apparent that the light green and the dark green sections are the same thing until you realize it's an "O" from "Google", and actually adds no information. Second, what do the size of the circles represent? Is it combined daily spending or average daily spending per advertiser? It takes a while to find the circumferencial text, which you'd guess that it represents the amount of revenue from top N advertisers. Then the chart also mixes terminology. While spending by advertiser and revenue by google are the same thing, you need to do extra work to figure that out. Then, what the heck, the list of logos on the side is distracting. It's suppose to be the advertisers in the blue circle--the top 10 advertisers--but it sits firmly in the red section, which is the long tail of advertisers. Even more confusing, the $59,184,783 is red, but points to the blue list of logos. Lastly, the average daily spending is colored with the same position and weight as the combined daily spending, but it doesn't represent the size of the circles, which adds even more confusion.
The only thing they did right was to match the size of the circles with the amount of combined daily spending. Often times, people will draw these sorts of graphs using the diameter as the basis for comparison, which is misleading.