Dark Sky is kind of an expensive app, as apps go: $3.99. Which means we have a small percentage of the market, giving it a very different growth curve from free apps. Our rate of revenue has been slowly increasing since we launched. And since our market share is so low, I don't see us running out of potential customers any time soon.

(And to answer your other question: Forecast.io makes some money, but not much. And we do have an increasingly large revenue stream in our data API)