There are a number of ways to discuss API popularity. Our metrics are based on developer engagement. The "track" functionality on ProgrammableWeb lets developers declare an interest in receiving updates on particular APIs. Which APIs and categories are seeing the most activity.
GoodGuide is a service that we have been using as a family for a while. It attempts to analyze a product holistically & scientifically to produce a simple 1 to 10 rating of its "goodness." I am personally satisfied to see that the GoodGuide API will share this data with the world.