open API that is making the service more relevant to users by spawning a whole ecosystem supporting its use and making the experience richer for all participants. By one estimate, there are now 50,000 twitter apps and there is 20 times the volume of requests to the twitter API as there is to the web server. Similarly, the facebook API is distributing facebook data on the web and throwing off its own ecosystem - sometimes interlinked with twitter, sometimes not.
Are any other services similarly spawning partner apps to enliven the core experience? One social networking app I've been using recently is Foursquare, and I just found Wheredoyougo.net which can generate a heatmap of your activity using the Google App Engine for its mashup. Seems like I'm basically a Northsider.
The appearance of Wheredoyougo.net is a strong indicator to me that Foursquare is going to have a strong set of partner apps in 2010, justifying some of the hype going around that foursquare is the next darling of the internet cognoscenti. And this differentiates foursquare from services like Gowalla, which seem to lack that developing series of partner apps.