The busiest shopping mall out there is Facebook?

True -- there are more than 250 retailers with pages on the social networking site, and surely there will be more to come.

And it's the most cost-effective advertising on the planet -- "fan" pages are free, as are many of the ways to use them, from announcing launches and events to posting comments and images.

Consider this stat: For every 10,000 fans a brand has, Facebook estimates it reaches 1.5 million people (a fan has an average of 150 friends who all receive updates and comments from the original fan).

Furthermore, retailers like Topshop are reporting that more than 10 percent of the visitors to their websites are linking from Facebook.

Sephora ran a campaign on Facebook with ads, a fan page and a coupon code and doubled its sales during the campaign, WWD reports.

With more than 87 million users in the U.S. and more than 300 million worldwide, the potential to reach consumers through Facebook is almost limitless.

Louis Vuitton attempted to harness some of that power when it broadcast its spring fashion show live on the its Facebook page, titled "Art of Travel by Louis Vuitton."

The luxury brand's more than 784,000 fans generated thousands of "I like this" votes and several hundred actual comments that created automatic and almost instant feedback the company could translate to its business.

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