This farewell is a truncated version of the expression "It's been real, it's been fun. Just hasn't been real fun!" It is not necessarily a slight to the person you are leaving, but more of the situation or experience that you have just shared.
While being dropped off after a car ride with a friend during which you helped him change a flat tire and ran out of gas - you turn to him and say"It's been real!"
A phrase used as a farewell, when the experience shared has been of unique, experimental, or strange phenomenon. Also, a manner of stating that surreal time frame has been shared between two or more peeps.
johnny; "I am outtie to AZ. Peace, it's been real."