All of the sudden Foursquare's success is starting to make a lot more sense to me. Apparently they have opened their location database to developers via an API. What this means is that other application developers can utilize the location database built by Foursquare when needing to include location check-in services in other apps. They recently reached the milestone of 10,000 developers using their database - that's quite meaningful. The article notes that the biggest user of the API is Instagram - an extremely popular and fast growing photo sharing app exclusively on the iPhone. I'm not sure what Foursquare charges for API access, but if they're charging I'm sure it's a meaningful component of their revenue stream.