Facebook adds an auto-enhancing feature for pictures taken and uploaded from mobile apps. The enhancing can be altered or removed when the picture is captioned or the feature can be disabled altogether. Facebook director of engineering Brian Cabral explains that the feature is meant to be a clean up tool, only.
Pragmatically this means you can see something in shadow and bright light when both are in the same scene. Even though your eye is adapting as you move your eye around, you remember the whole scene as if it was evenly lit.