FrontierVille," a Zynga game that is exclusive to Facebook, allows you to send links to your friends as an incentive to get them to join the game. In return, they help you get an item in the game that you would otherwise have to pay for.
Instructions:
1) Browse to the item you want in the store.
2) Click on the "Invite" button.
3) Click "Ask Friends."
4) Select the friend you want to send the link to, and they will get a post on their wall detailing how they can get you the item for free.
Source: eHow
