Build a Pay by Social Networking Post, Platform

Need a platform which will allow marketers to pay for a product or service by tweeting or posting a pre-determined message to thier Social Networking Account. Once the post or tweet has been made than the visitor would gain access to the free product or service. Something simple; only need three social networking sites; Twitter; Facebook; LinkedIn. Keep the price as low as possible; don't want to freighten the client away. Will only provide a 25% up front payment; 25% halfway through project and the balance once it is proven that the platform works without any bugs. It can be built for a WordPress site, or not. Thank you.