Looking for your friend? If the people in your social circle use Plazes, a Swiss location-tracking startup, they may not be too hard to find. Plazes is an online community where people post and update their location via the Internet or text messaging on cell phones. The company has been around since 2004, but in the past month, it has revamped its product, making it simpler to use and letting people upload pictures of different locales and add reviews.

The idea of updating friends about your activities and location has gained traction over the past couple of years, and a number of Web-based services have cropped up to support it. Twitter is a micro-blogging service that lets people update others about their activities from their phone, a website, or an instant-messaging client. Jaiku has much of the functionality of Twitter, but it lets users add the element of location to their posts. Dodgeball, owned by Google, is a text-message-based service for posting location information for friends. And Sprint provides a service called Loopt that enlists global positioning sensors to locate users, and it displays a map of their whereabouts on mobile phones.

“There is a sense that it’s important to find friends and share location with each other,” says Mor Naaman, a research scientist at Yahoo. Indeed, an increasingly common opening question in cell-phone conversations is “Where are you?”

Felix Petersen, cofounder of Plazes, hopes that his service will be the easiest way for people to answer that question. When Plazes was relaunched last month, the company introduced some changes to try to make the service accessible to nongeeks. Initially, people could only “plaze” themselves using a computer at a wireless hot spot. Today, they can also text-message Plazes with their location. People can now upload pictures of places, rate locations, and subscribe to people or places to get any new, related posts. In addition, Plazes lets users post their location to Google Earth and integrate it into blogs and Facebook profiles.

One of the values of Plazes, says Petersen, is that it can act as a more personalized recommendation system. Yelp is a service that lets people post reviews of restaurants and stores, but in general, the people who write the reviews aren’t friends of the people reading them, so it’s hard to gauge which reviews to take to heart. With Plazes, a person can learn what a friend thought of a place, or simply how often he or she visits. Additionally, since a user’s location information (either present or future) can be broadcast, friends can chime in with suggested places to visit nearby. Before a recent trip to San Francisco, Petersen posted this future location to Plazes, and his friends helped him find an inexpensive, charming hotel.

Plazes, as with other emerging location-tracking services, has a number of obstacles to overcome. To start, the Plazes community is relatively small, with fewer than 100,000 active users. The more users a service has, the more likely there will be relevant information about a larger number of places. While Petersen says that Plazes is growing, the number of users is tiny compared with online communities such as Facebook (which has tens of millions of users) and MySpace (which has hundreds of millions of users).

Another hurdle is balancing privacy. “There are worries about privacy in all the systems,” says Yahoo’s Naaman. “There’s obviously risk in sharing location with the public or even other people.” He says that best practices for such systems need to be established so that people have a better understanding of how much they can and should disclose, and to whom.

Plazes’s approach to privacy, says Petersen, is to let users determine if their locations are public and which friends can see them. In addition, people must manually “plaze” themselves in order for others to know where they are. “People publish their locations,” he says. “They have to initiate it.”

Petersen notes that he and his team are still looking to make Plazes more user-friendly. In the coming months, people will be able to update their location using e-mail and instant-messaging tools, he says. Also, the company will experiment with location-based advertising–a sort of “AdSense for the real world,” says Petersen. He envisions landing at an airport, posting his location, and getting updates on rental-car deals or coupons for services that he used when he was last in town. Plazes can provide a fine-grained level of filtering so that ads can be targeted precisely, he says. At that point, “it’s not advertising anymore: it’s information.”