While SMBs can't afford to pay big bucks to advertise their wares in traditional media like TV, magazines and billboards, the Internet can help them level the promotional playing field.

SMBs, which already are fans of Google's AdWords and pay-per-click advertising, will also likely benefit as the search giant and other companies work to enhance their local advertising capabilities. And social networks are an especially intriguing avenue, since they essentially amplify the effects of the word-of-mouth advertising that has served many SMBs well over the years.

One such network that itself appears to be breeding viral interest is Yelp, which Fortune describes as "Facebook meets Zagat." The owner of an Orange County, Calif., hair salon tells Fortune he is "making a ton of money from Yelp, and it's freaking me out."

Yelp CEO Jeremy Stoppelman says the site is far more democratic than, say, the Yellow Pages. "If you look at the yellow pages, what are you seeing? You're seeing how much money a business spent to buy a big ad. We're a place for a conversation between the prospective customer and the business owner."

In a sign of how effectively Yelp is capturing the public zeitgeist, folks have taken to using it as a verb, a la Google. Example: "I Yelped that place with the great hummus." Regular users are known as "Yelpers."

As with other sites that provide customer reviews, Yelp can also serve as a kind of customer focus group. "Your customers are out there saying things about you, whether it's on Yelp or on some blog. The faster you can fix problems, the better you're going to do. Customer service is the new marketing," says Stoppelman.

There are some caveats, of course. Business owners have little recourse against Yelpers who fabricate reviews or otherwise use their powers for evil rather than good. And there are questions about just how much influence social networks have over buying behavior.

Still, SMBs likely have more to gain than to lose from sites such as Yelp.