Every once in a while, there’s a moment that reminds us of just how rich Amazon is.

Thursday afternoon, they announced that they’d earned $200 million more in 2010 than they had in the previous year. (“Net sales increased 36% to $12.95 billion in the fourth quarter, compared with $9.52 billion in fourth quarter 2009.”) In fact, it was the first year that Amazon’s sales were more than $10 billion for a single three-month period. Amazon’s CEO said they’d sold “millions” of Kindles in those 13 weeks, and then he dropped an even more stunning peice of information.

“Kindle books have now overtaken paperback books as the most popular format on Amazon.com.”

Amazon had announced last July that they were selling more ebooks than hardcovers. But at the time, I’d complained that was misleading, since hardcovers make up a small percent of total book sales at any store. One analyst had calculated that there’s usually three paperback books sold for every one hardcover book. Combining that information with Amazon’s statistics, it seemed like in July Amazon’s ebook sales were only 54% of their paperback sales.

But not any more. In fact, Amazon explained today that for every 100 paperback books they’ve sold this January, they’re selling 115 ebooks. That’s another way of saying that ebook sales have risen to 115% of Amazon’s paperback sales — that is, nearly double what it was in July. That’s even more impressive than it seems, because paperback sales are actually increasing, according to Amazon’s announcement today. And they’ve sold “three times as many” Kindle ebooks as they have hardcover books, according to today’s announcement. If you graph it all on a pie chart, it looks like this.

Of course, that still means that Amazon is selling fewer ebooks than they are printed books — if you combine the paperback and hardcover sales. But ebooks now represent more than 45% of all the books that Amazon is selling. If ebooks can just increase their share by 5%, Amazon will finally be able to announce that they’re selling more ebooks than all print books combined. And that day could come sooner than you’d expect. Amazon predicted last summer that ebooks wouldn’t start outselling paperbacks until at least April of this year. They beat their own prediction by at least three months!

Of course, it’s possible that this is a one-time spike. (After all, there were a lot of new Kindles that were activated on Christmas day.) It’d be interesting to see whether ebook sales actually drop below paperback sales again at some time during February or March. But Amazon’s figures are even more impressive when you realize that not every printed book has an ebook edition yet. And to achieve this milestone, Amazon didn’t even count any of the free ebooks that people are downloading, which is presumably an enormous number.

In fact, if just one user downloads a free ebook for every nine paid ebook purchases — then Amazon is already delivering more digital ebooks than they are print editions!