Loopt, a mobile service that allows users to track friends on a map and communicate with them, is now available for BlackBerry users on Alltel, Sprint, T-Mobile, and AT&T Networks, the company said Monday.

After downloading the free program, subscribers can set up a private network of friends they want to be able to locate on a mobile map. Using the handset's GPS, Loopt essentially creates a mobile social network that works across multiple devices and carrier networks.

Loopt users can be alerted when a friend is nearby, post and share geo-tagged photos, read and maintain a mobile blog, and comment on others' uploads. For privacy concerns, users can turn these services on and off on a friend-by-friend basis or for all friends at once.

The company was in the news on June 9 by demonstrating its service on the iPhone during Apple's Worldwide Developers Conference keynote. While the iPhone 3G may capture the headlines, a recent IDC report said Research In Motion's BlackBerry still dominates the smartphone market.

Loopt is now available on RIM's BlackBerry Curve, Pearl, and World smartphones. It's also available on multiple Sprint Nextel devices and some Boost Mobile handsets, and will be featured on Verizon Wireless cell phones in the future. A full list of devices can be found on the company's Web site.

As more cell phones feature integrated GPS capabilities, the market for location-based services is poised for rapid growth. According to a recent report from ABI Research, revenue from this market is expected to rise from $515 million to $13.3 billion in the next five years.

While enterprise applications like workforce tracking are expected to produce the most revenue, the ability to find and interact with friends could be a valuable market. Loopt is already facing competition from other location-based social networking services like Brightkite and Whrrl.