Discount in Virtuemart by Tweeting and FB sharing

Hello all, I am looking for a application/module in Virtuemart so that if customers press on a "tweet" or "share" button they get a discount for tweeting/sharing it. So the price immediatly becomes % lower than normal. This will stimulate the Word of Mouth of my store. Example: People goes to a productpage, he clicks on "tweet this" the page redirect him to his Twitter account with a default text "I bought [product name] on [website]!". After tweeting that he returns back to the product page and gets % discount on the product. Same for FB. Thanks.