How to setup an agile startup without the customer?

In agile, there is an emphasis on customer involvement. They provide user stories, provide feedback, and a lot of decisions are up to them.

There are situations that there is no customer for the software at the time of development. If we are going to ship a game -a software that people will be its customers after shipment- or if we are going to create a new service -like a new social network- and we are going to own it, there won't be any client present at the time of development.

I have a few questions for these environments:

1. Is there any software development methodology/framework specially designed for such cases?


2. Is agile applicable to such environments? If yes, How?