Klout‘s API served over 2 billion API calls in June, according to the company blog. Klout launched its API before it even launched its website. But this number pales in comparison to Twitter’s 15 billion calls per day. In fact, Twitter serves more API calls per day than both Google (5 billion per day) and Facebook (also 5 billion per day) combined.

The Twitter app ecosystem exploded, increasing sixfold in the last year, and these API numbers illustrate the effects of that growth. Part of the reason for this growth, as our Marshall Kirkpatrick wrote recently, is that there’s been a lack of social data for various social media analytics companies to mine. That might change when Google Plus opens its API, but for now Twitter is the best game in town for many developers.