Github makes it easy to start participating to a project. The recommended way of doing this is just fork, push your changes and send a pull request to the origin project. So the question is now, how are projects using this workflow, and which projects are relying on it.
Note: all along, I use a kernel of query to extract pull requests related to forks. The request is limited to one month to keep it under the size limit. As an added bonus, the query provides the average latency between fork and pull-request.
The query named Fork2PullRequestByProject.sql helps sort out the greatest projects using of this feature over the month of april: