In 2003, The Neptunes produced 43% of songs on US radio, and 20% on British radio.
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50 Mind Blowing R&B Music Facts

45. In 2003, The Neptunes produced 43% of songs on US radio, and 20% on British radio.
(via http://www.theage.com.au/articles/2004/05/12/1084289744331.html)