Have you noticed the many tools to share your location? There's FourSquare and Gowalla, both about a year old. Then there are old-timers like Brightkite. And now, there's one that works with all three (and a couple more).
A year ago Twitter was just a microblogging platform. Sure, it was a popular one, but it wasn't until the announcement of its geotagging API that it took its first step toward being a location platform. Since then, it has expanded its offering to include the four geo APIs I've categorized below.