It is simply not part of the standard operating procedure of a major power like the United States to authorize its friends to take rapid and unilateral military action.
This new wave of impromptu-activity apps will have to compete against the gold standard of location-based services: Foursquare, which is a free smartphone app that tells friends where you're hanging out.
In a world in which an online presence is becoming a standard accompaniment to real life, online communities and presences can quickly go from feeling limited to close friends and family to a place where, should they want to, a foreign government can pop in and say hello.