Over all our clients in aggregate, in July 2008 Google received 80% of our clients' ad dollars. Yahoo received 15%. Microsoft received 5%.
Put another way, Google continues to control over five times the paid search market share of Yahoo.
Remarkable how stable these aggregate percentages are, despite continuous change in underlying data as our agency launches new clients, clients shift strategy, summer moves toward fall, etc. A Law of Large Numbers effect, clearly.
For full details, see June 2008 Google, Yahoo, Microsoft Paid Search Market Share.