I was thinking about building a service on top of BrightKite, but at 1/20th the number of visitors vs. Twitter I'm not sure. Are there any other location-based

I was thinking about building a service on top of BrightKite, but at 1/20th the number of visitors vs. Twitter I'm not sure. Are there any other location-based social networks with a published API? Loopt seems to see itself more as a program and less as a platform.