As an undergrad at Stanford, Sam Altman was struck by how his fellow classmates increasingly relied on cell phones to communicate -- especially text messaging. A texter himself, he began to realize that the most common message people tended to send was pretty basic: "Where are you?" So he decided to build a company around this very question.

The result was Loopt, a social mapping service for cell phones, which has become a rising star in the tech world.

A computer science major, Altman's goal was to create a better way to connect with friends via cell phone. While he had no formal business plan, he assembled a team of classmates at Stanford and built a prototype. With a working model in hand, he then started pitching investors, and ultimately Loopt issued a round of Series A stock, raising $5 million from Sequoia Capital and New Enterprises Associates.

Then came the hard part -- finding a telecom company to work with. "We were a very little company trying to do business with these behemoths, and it was really intimidating," says Altman, now 22. Loopt eventually landed its first partnership, with Boost Mobile, a subsidiary of Sprint Nextel (NYSE:S).

How does the technology work? Loopt users make a list of friends who also have the service, and the program allows them to see one another on a GPS-enabled map. At its simplest, Loopt at is a friend-finding service. But other features, such as text alerts, make it a true social networking tool. "It's actually one of my favorite features," Altman says. "You get an alert when a friend of yours comes within a distance you set. So I can know when 'Mark' comes within a half mile of me. And when he does, I get a text message and I can go meet up with him."

The company's focus is to bring the digital world into the real world. Users can create events, tag them to a location, and send invitations to friends. Or they can keep a journal, with short blog entries tagged to specific locations or photos. But for those reluctant to have a constant homing beacon on their cell phones, the program comes a host of privacy features -- allowing users to block friends from seeing their current locations, or turn off the automatic updates altogether.

In June, Loopt announced it had reached 100,000 subscribers. And even though the service, which runs about $3 a month, only started on Boost, it will soon become available to all 56 million Sprint Nextel users -- with plans to add other carriers in the near future.

The world may be embracing social networking on the Internet, but Altman feels the technology has its limits. "There is a lot of buzz about online communities and that's really cool," he says. "But if you're stuck in front of your computer and I'm in front of mine, we don't have much real-world interaction." Altman believes that Loopt is filling that void. "The company's mission is to enhance, improve, and make more of the real world."