Google's mobile ad network AdMob receives 2bn ad requests per day

"The AdMob network now receives more than 2 billion ad requests per day.  This represents an increase of more than 4x in the last twelve months.
Nine countries in the AdMob network generated more than one billion monthly ad requests in December 2010, up from one country just a year ago.
The 10 countries in Western Europe with the most ad requests all experienced a greater than 400% growth in ad requests in 2010.
More than half of AdMob's top 50 brand advertising campaigns in December 2010 used the AdMob Interstitial Ad Unit (NEW)
Ad Impressions for AdMob grew more than 500% in the past year from the AdMob Interstitial Ad Unit (NEW)
There are now over 50,000 publishers in the AdMob network that support their mobile businesses with mobile advertising (NEW)
Hundreds of mobile publishers, including Angry Birds, are earning over $100,000 per month from AdMob ads."
Source:  Google Mobile blog, 31st January 2011