Sorry for so many business posts! This is what happens when I am well rested and full of Turkey :-) Scott Switzer suggests that you ought to develop in this order:

API First, Mobile Second, Web Third

He claims that the most important consumer of your product in this age is not necessarily a person, but a machine. I agree with him from a scale perspective. Thinking about how your product can add value to other products and services is the fastest way to grow. However, I would recommend approaching things from the opposite order when designing the product. Choosing the ultimate use case with a human in mind (usually yourself) is the best place to start.

Google Maps has to be one of the best examples on both ends.