"Where are you?" It's the most common question asked by mobile phone users. Loopt announced Tuesday that Sprint will launch a service that answers that question by using GPS capability to enable subscribers to see where their friends are located.

The social mapping service automatically updates the location of other subscribers in a user's private network; the location information is displayed on a map on users' phones. The locations of subscribers are updated every 15 minutes.

The service costs $2.99 a month in addition to Sprint's data charges and will be available on 25 Sprint and Nextel phones in the coming weeks, Loopt said.

According to the Loopt announcement, the service "allows users to send messages to groups of friends or send proximity messages to friends when they are nearby."

Social networking systems on mobile devices have been catching on in recent months. A system called iFind at MIT is device-centric and secures information from users outside the application. The MIT system runs off the university's massive Wi-Fi network.

Most mobile phone location services currently use GPS technology and provide navigation information. Sprint already offers a service that enables parents to keep track of their children and another service that helps runners stay on track while training and racing.

The Loopt service can send an alert when another subscriber is nearby; messages also can be broadcast to selected groups of users. The service has been available on Sprint subsidiary Boost Mobile for several months and more than 100,000 subscribers have used the service since then.

Loopt said its service is 100% permission-based, with users agreeing to share their location only with their designated friends; the service can be turned on and off on a friend-by-friend basis or for all friends simultaneously.