FarmVille" is an online game playable through the social-networking website Facebook. Like a number of other Facebook games, the objective is to grow your farm with the assistance of your Facebook friends. In order for your friends to help you, however, you must add them as neighbors to your farm. The only way to do this is through the game, but it takes only a moment to complete.
Instructions:
1) Log into Facebook and click "FarmVille" on the left panel.
2) Click "Add Neighbors" at the top of the FarmVille screen.
3) Click a friend in the "Friends" box on-screen. A friend will only be listed if that person already plays Farmville but is not currently your neighbor.
4) Click up to 40 names and then click on "Send Farmville Neighbor Request."
Source: eHow
