No, maintaining possession to the ground is not the rule. It should be the rule, but its not. That is my gripe. You are required to maintain possession even after to the ground. If you have possession, and and any part of you other then the face of your hand touches the ground (just like a ball carrier)you are down at that spot.
If you compare this rule for a receiver to a player running with the ball it would be like if you were tackled and dropped the ball after your knee touches the ground and it being ruled a fumble because you didn't maintain possession thru the process of being tackled.