The above chart is from the Interactive Advertising Bureau's October 2012 report (pdf) on internet advertising revenue. What it shows:

Online advertising continues to remain concentrated with the 10 leading ad-selling companies, which accounted for 73% of total revenues in Q2 2012, up slightly from the 72% reported in Q2 2011. Companies ranked 11th to 25th accounted for 9% of revenues in Q2 2012, consistent with the 9% reported in Q2 2011. Companies ranked 26th to 50th accounted for 8% in Q2 2012, also consistent with the 8% in Q2 2011.