Note: development of geojson.io is currently paused. You may be interested in the new fork at https://geojson.net. Until development restarts, please refrain from adding issues to the tracker.
FAQ
Q: Why are my coordinates flipped?
A: In the GeoJSON format, longitude comes first, before latitude.
Q: Isn't that wrong?
A: No, almost every kind of spatial file that exists, including KML and Shapefiles, does the same thing. Math also tends to tell things in X, Y order, and we usually size things by width and then height.
Q: Why does geojson.io require access to private repos?
A: Because if it didn't, you wouldn't be able to open files in your private repos via GitHub.