When the Kindle sold out during the holidays, I guessed that Amazon would end the year selling 500,000 of its electronic books. All I did was roughly double the 240,000 that had sold through the middle of the summer. But now Citi analyst Mark Mahaney has come to the same conclusion, using better data. In a note today, he cites some numbers in Sprint's 10Q filings that indicate 210,000 devices were activated in the third quarter, and 100,000 each in the first and second quarters. (Each Kindle downloads books wirelessly using a built-in Sprint EVDO antenna). In addition to the 410,000 activated Kindles during teh first three quarters, he estimates that Amazon shipped a total of 500,000 activated Kindles before selling out in mid-November. (Oprah had something to do with that). If it hadn't sold out, Mahaney thinks Amazon could have sold 750,000 Kindles in 2008.