When someone walks in to The Coffee Groundz in Houston and demands a free coffee because they are "mayor," general manager J.R. Cohen asks to see the person's cell phone. If that makes any sense to you, you're probably familiar with the urban social media phenomenon foursquare.com.

Foursquare is best described as a mashup of Facebook, Twitter and real-life meet-ups. Users can "check in" from their locations and also see where other Foursquare contacts are lurking. The person who checks in the most from a particular spot is named "mayor" of that spot. "We have people challenging each other all the time to be mayor of Coffee Groundz," Cohen says. "We've definitely seen an increase in business, and offering something free to the mayor has become a great game."

It's yet another brilliant marketing strategy to emerge from social media: In addition to Foursquare, Twitter users "tweet up" to get together and Meetup.com is all about organizing the face-to-face -- which makes them all opportunities for businesses to attract more traffic, says marketing consultant Alan Wolk, who blogs at tangerinetoad.blogspot.com. Wolk says the key is to get in the conversation and offer something to those potential customers.

"If you can offer something of value in exchange for meeting up at your place of business -- maybe a free drink if you own a coffee shop or a free personal training session if you have someone who checks in a certain number of times from your gym -- then you can use these tools to increase your traffic," he says.

Meetup also allows businesses to sponsor groups, offering them everything from financial support to a place to meet. And Foursquare has an alert to let people in the area know when a business nearby has a special offer for them.

Gwen Moran is co-author of "The Complete Idiot's Guide To Business Plans". Reach her at gwen@gwenmoran.com.

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