Amazon does not release Kindle's sales numbers, but here is an article that attempts to speculate at those numbers broken down on a rolling annual basis.
http://seekingalpha.com/article/246700-just-how-big-is-the-kindle-revolution
*From the article
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Kindle Launch - November 19, 2007
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100,000 Kindles - March 2008
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750,000 Kindles - October 2008
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1 Million Kindles - March 2009 (Kindle 2 Ships)
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3 Million Kindles - December 2009
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4 Million Kindles - July 2010
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6 Million Kindles - August 2010 (Kindle 3 Ships)
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8.5 Million Kindles - December 12, 2010
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11 Million Kindles - December 24, 2010
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12 Million Kindles - January 2011
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22 Million Kindles - December 2011 (conservative projection)
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35 Million Kindles - December 2012 (conservative projection)