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Who is going to use your application? With more than 800 million users actively using Facebook in over 70 languages, it is important to understand who the intended end user is for the application. This should not be a difficult task, just an exercise in common sense to help you gauge your expectations. For instance, if you are building a set of puzzle games that makes the Sunday New York Times' puzzles look like child's play, it may not hold the attention of 8–10-year olds for long. If you really want to build something enjoyable for the elementary school crowd, you may need to rethink your idea, or shift your thinking in audience to what actually fits your idea.
A big issue you may run across if you are doing contract work is helping guide your clients in identifying their audience. You may get a response such as “Everyone should be able to use this application,” but in reality, this is infeasible. You need to be able to help your client refine the “everyone” statement to a specific group of individuals who you want to attract.