Assuming that your daughter is OK from a medical standpoint, such behavior is an example of obtaining pleasure in a fashion that should be reserved for private time. It won't be helpful to characterize the behavior as bad, but rather educate her about where and when it's appropriate. I think you'll notice it will wane over a bit of time.
Well i have a friend that did this when she was little now she talks about it a laughs. but being a mom to a girl and a boy and two step kids i listen carefuly. her mom used to tell her that she needs to do that in her room by her self. she supported her and never made her feel as if she was doing anything wrong. she ended up growing into a girl that knows her body and did not go to men for pleasure she was the only girl i knew that waited intill marrige she's 35 and happy that her mom let this go on.