Take a look at the Comscore rankings for December 2011: Major brands are rapidly becoming publishers themselves. Amazon, E-Bay, Wal-Mart, Sears, Target, Best Buy, and AT&T have all moved into the top 50 U.S. online publishers. Of those seven, Amazon, E-Bay, Wal-Mart and Sears are already running ads on their websites.

It is true that these brands are nowhere close to the Facebooks, Googles, Microsofts and Yahoos of the world when it comes to advertising impressions and, no doubt, advertising revenue.

But these brands have been moving up the leaderboard for months and are now giving a number of other online publishers a serious run for their money, at least in terms of unique visitors and page views.

Consider that the seven brands listed above now represent 14 percent of the publishers in the Top 50 publishers in the United States, and all of these brands were basically nowhere to be found in the rankings a year ago.

In December, each of them finished higher in the Comscore rankings than premium advertising sites like Yelp, Scripps, Fox News, the Washington Post, IGN and the NFL.

It may seem obvious at first blush, but it makes perfect sense for brands to leverage their massive audiences to become advertising-supported publishers. These brands have hundreds of millions, if not billions, of advertising opportunities on their Web pages every month. Why not take advantage of those opportunities and pick up what could be found money?

Amazon, for example, has made a business out of listing and selling a wide range of products. Why shouldn’t it sell advertising on those same product pages? It would be kind of like those “end caps” at the supermarket: close to the context. Without the ad, maybe you wouldn’t have thought to buy that salsa otherwise. Let the best brands win, or at least, extract money from other brands to prominently advertise their wares.

And I’m sure that the big e-commerce sites like Amazon, E-Bay and Orbitz have figured out that the margins on advertising are a whole lot healthier than the razor-thin margins they get from selling retail products.

It would be foolhardy to think that advertising revenue on e-commerce sites would replace the product revenue anytime soon. But accepting advertising would provide these sites with another revenue stream and one with very high margins.

It is also worth pointing out that brands usually have a fair amount of first party data about the visitors to their web sites. Setting aside privacy issues, why wouldn’t Bank of America target Mercedes ads to its high net worth individuals and KIA ads to its savings account customers?

The democratizing force of the internet forced traditional publishers to scramble to compete online with new digital publishers who built their brands purely in cyberspace.

Yahoo owned the online news space more than a decade before the New York Times created digital subscriptions. The Huffington Post perfected the art of aggregation and community (some publishers would have other choice words for what Huffington did). And Pandora flipped the iTunes revolution with the oldest broadcast medium that still exists, offering music lovers the chance to create “ideal” radio stations.

So here’s a fresh example of the internet’s law of unintended consequences: corporations who never had an interest in publishing may find that there is money to be made by sharing their customers with other brands.

It’s not exactly Macy sending you to Gimbel’s, but it’s pretty damn close.