The company: Kogi Korean BBQ, a year-old Los Angeles start-up that operates four roving food carts. The grub is Korean-style meat served in Mexican-style flatbread. Dishes include Korean short ribs tacos ($2) and spicy pork quesadillas ($7).

The idea: Kogi uses Twitter to tell customers where its trucks are. Except for a few regular spots—like Little Tokyo on Thursday nights—the trucks move every couple of weeks, and managers will sometimes make a last-minute change if the trucks can't find a good spot to park or if the location seems deserted. The idea is to cater to a larger customer base than would be possible with a few regular locations. "We realized that if we wanted to move around a lot, we needed something like Twitter," says co-founder Alice Shin. Customers—some 50,000 so far—sign up to follow Kogi's Twitter feed and get updates about locations, as well as specials and random jokes from Shin.

The result: Extremely rapid growth. Kogi now employs 60 people, up from just four a year ago. "We've gone from selling 30 pounds of meat a day to 1,000," Shin says. At any given time, about 10 percent of the customers learned about the location of the truck from Twitter. The rest, Shin says, "see the line and assume it's good." And if nobody seems to be interested in buying, the truck can always go somewhere else.

How to get retweeted, Part I: Don't shill, and don't spam—or your followers will revolt and stop following. Companies sometimes hire Kogi's trucks for promotional events and then ask Shin to fire off an advertising message to Kogi's followers. Her policy is to decline these requests. "If you spam people with advertising, it'll backfire in the end," she says.