In that case you don't have a customer, you have to dog-food it. Most of the time when agile references a customer, customer feedback, and customer interaction it's actually the customer contracting the work. That is a different context than a startup or video game developer or others because there is a clear customer stakeholder who can provide feedback and validation from the start.

This gets to part of the problem in all these discussions. The context that the Agile Manifesto was written in was not in creating Facebook or Google, but in creating software for a defined customer who held a stake in the outcome (had invested capital, along with wanting/needing the results). Once written, something like your Google Docs replacement can get a customer stakeholder, but they won't have it at the start unless you line up early users (and even then, they probably aren't invested).