Electronic retailing giant Amazon.com has never revealed how many units of its Kindle ereaders it has sold, always offering vague glowing statements that the Kindle is the company’s “most gifted” item and that Kindles continue to set sales records. However, without knowing how many Kindles the company selling, all those statements are almost meaningless: after all, if you sell three devices on quarter, selling four devices the next quarter is a 33 percent increase in sales!

Amazon dropped a hint about how many Kindles it is selling: the retailer claims to have sold “millions” of Kindles in the first 73 days of the holiday quarter, and furthermore that holiday quarter sales to date surpass the total of all Kindle sales for 2009. It’s still not a specific number, but it means that Amazon has sold at least two million Kindles so far this quarter—and that number is only going to go up as the end-of-year holidays loom closer.

Third-party estimates of Kindle sales for 2010 have generally hovered around the 4 million unit mark, although Citigroup’s Mark Mahaney recently increased his estimate of 2010 Kindle sales to 5 million, and forecast Amazon will move 8.4 million Kindles in 2011.

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