The (SXSW) festival this week in Austin has provided a platform for to demonstrate its social networking application, in which users give their friends permission to see their whereabouts after they "check-in" at specific locations.

This is not a space Foursquare has to itself. , , , , , and already have location-based apps or features. has added location data, Google offers its Latitude service, and Facebook is location information to its service.

Foursquare offers obvious marketing potential for businesses that can mine the check-ins and turn them into a reason for friends to gather and spend money. I am imagining restaurants, bars, coffeehouses, and other places where people congregate, though other businesses might be able to profit from the scheme as well.

Perhaps the first person to check-in at a location might get a discount or freebie of some sort, one that grows as more people from their cluster of friends join them. I suspect there are places and businesses where this could work extremely well.

My nightmare scenario is of advertising personalized to my interests as well as my location. Imagine a world where as you drove down the freeway every billboard would be targeted, and very precisely so, directly at you.