"NASA's Curiosity Mars rover checked in on Mars Wednesday using the mobile application Foursquare. This marks the first check-in on another planet. Users on Foursquare can keep up with Curiosity as the rover checks in at key locations and posts photos and tips, all while exploring the Red Planet. "NASA is using Foursquare as a tool to share the rover's new locations while exploring Mars," said David Weaver, associate administrator for communications at NASA Headquarters in Washington. "This will help to involve the public with the mission and give them a sense of the rover's travels through Gale Crater."
Keith's note: What PAO hasn't explained is why Mars Exploration Rover Opportunity hasn't already checked in on Foursquare. After 8 years it should be the mayor of Mars.