The list of your neighbors is displayed along the bottom of the game area. You may send Neighbor requests to any of your Facebook friends. Those who accept the request become your in-game Neighbors.
When you perform actions in neighboring cities, their buildings drop 1 fame, 1 gift box, and a chance at Simoleans, Resources, and collectibles. Each gift box moves you on the relationship status bar at the top of the game area. When the status bar fills to the right, you move to the next friend or Twin City relationship. When the status bar empties, you move to the next foe status.
Higher relationship levels increase the probability of dropping collectibles when you perform actions on neighboring buildings.