You don't become listed as a contributor by merely forking a repo, your fork is just included in the original project's "forks" count.
The project is included in a user's "Repositories contributed to" area when that user creates a pull a pull request or files an issue.
You are listed as a contributor on a project's page only once you have had an accepted pull request.
EDIT: take one of my projects for example[0] - 152 forks recorded but only 8 people are listed as contributors.