Admob released some real numbers around their traffic today. They are closing in on delivering 1.5 billion ads per month. More than a third of this traffic is in the US. Some of the growth has been organic while some has been driven by adding partners to their ad network. It's impressive either way.
Some of the challenges advertisers face when evaluating new mediums include the ability to understand the impact of their advertising along with the audience they are reaching. Admob has published some interesting numbers that demonstrate that they know who their audience is (beyond the expectations of a web property given the content) as well as who is browsing. I suspect there is a lot more data where this came from. This is good news for advertisers and the industry – a better understanding of who is being reached will help drive growth in the industry overall. This kind of transparency and detail can be hard to get in nascent markets.