Mobcast is a free iPhone application that lets you share your location with your Facebook friends. The application can display your friends' location in near real-time on a Google Map and can also update your Facebook wall.
Using Mobcast you choose who can see your location using Facebook friend lists. It is also possible to choose how long your location is stored on the server after you stop running the application. You can even delete your location or run in 'stealth mode' at any time.
Get Mobcast for free on the iTunes Store