By Megan McArdle

Amazon.com Inc.'s shares fell almost 10 percent a few minutes after the company dropped some disappointing earnings news. The title of the company's news release is cheerily optimistic: "Amazon.com Announces Fourth Quarter Sales up 20% to $25.59 Billion."

And its operating income actually beat estimates — $510 million, compared with $489.9 million. But fourth-quarter sales of $25.6 billion were considerably below estimates of $26.08 billion, and earnings per share were 51 cents instead of the 69 cents that analysts had been expecting.

That's not just disappointing for Amazon; it's also not great news for the U.S. economy.

When retail foot traffic and sales were disappointing in December, the standard explanation was that people must be moving their purchases online. Obviously, they weren't — at least, not nearly as much as analysts expected.

Given how dominant Amazon is in e-commerce, this should cause most of us to revise our expectations of fourth-quarter retail sales, as well as growth in gross domestic product. And not in a good direction.

Megan McArdle is a Bloomberg View columnist who writes on economics, business and public policy.