Amazon earnings just came out. The company had a strong fourth quarter, with revenues up 42 percent to $5.7 billion, net income doubling to $207 million, and free cash flow doubling as well to $1.4 billion. On the earnings call, there is a lot of concern among analysts about margin pressure next year.

Most of the things that tend to interest us here at TechCrunch about Amazon are not yet material to its finances. But one detail that stuck out in the earnings release hints at the growth of Amazon Web Services:

Adoption of Amazon Elastic Compute Cloud (EC2) and Amazon Simple Storage Service (S3) continues to grow. As an indicator of adoption, bandwidth utilized by these services in fourth quarter 2007 was even greater than bandwidth utilized in the same period by all of Amazon.com’s global websites combined.

That means startups and other companies using Amazon’s Web-scale computing infrastructure now bigger collectively than Amazon.com, at least as measured by bandwidth usage. Amazon is one of the largest Websites in the world (No. 7 in the U.S.), so that is a significant milestone. Amazon doesn’t break out revenues from its Web Services, but presumably it is part of the “Other” line, which was only $131 million for the quarter and includes businesses besides Amazon Web Services (such as its merchant services).

Other tidbits:

—Registered developers in the quarter reached 330,000, a 10 percent increase from the third quarter.

—Demand for the Kindle continues to outstrip supply. CEO Jeff Bezos says on the call:

The Kindle, in terms of demand, is out-pacing our expectations. it is also on the manufacturing side causing us to scramble We are working hard to increase the number of units we can supply. Our goal is to get back into a situation when you order a Kindle, we ship it immediately. That is our standard.We are super-excited by the demand.

And in response to a question about whether there are hidden features yet to be turned on in the Kindle, Bezos punted by pointing out:

There are a number of experimental features. We have made them accessible so customers can tell us if we should work on them and make them part of the product.

The current “experimental” features are a basic browser, a music player, and the NowNow question answering service that uses Mechanical Turk in the background. Here’s my feedback: Keep the browser, and make it better.

At the very end of the call, Bezos was asked about the shift in media from physical to digital and how that will impact Amazon. Media accounted for 59 percent of Amazon’s sales last quarter (or $3.3 billion), and Amazon is moving aggressively to offer digital movies, music, and books. Bezos thinks this move to digital will eventually pay off:

When media was largely physical, it made sense to buy it in the physical world. But as media becomes digital it does not make so much sense to buy them in the physical world. The bulk of the sales now are in the physical world. So our relative advantage over time should improve.

Not to mention, it won’t have to take a hit on shipping costs.

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