FarmVille is a social game. Point. You can not progress in the game without neighbors. Even if you want to buy more land, you will need at least 8 neighbors before you may purchase it. Neighbors are a crucial part of playing, and enable you to do much more.
Neighbors can send you much needed gifts, especially in the beginning when you have little coins and have to wait for crops to grow before you can spend again. Very important gifts are animals and trees. In reality, these two specific items will cost you a fortune, and it will take months before you actually start profiting from it. It is just not worthwhile to spend your hard earned coins on them.
Beside gifting, neighbors can share their rewards with you. Most of the time, completing one of the ribbons will enable you to share your success with your friends, and they will be able to collect a bonus from you. The same is true the other way around. So if you need coins, collect from your neighbors!
It is also very important to have neighbors to adopt animals. There is a specific ribbon just for this feature. Sometimes, the adoptable animals are not available through any other way except through adoption.
You will also need to receive gifts from your neighbors in order to complete an achievement for your gift box. It takes a whooping 21 gifts to earn the blue ribbon, and each gift must be unique at that. It is no use having 10 of the same items at all.
And of course, the major ribbon of all, is visiting your friends to help them on their farms by chasing pesky racccons, squirrels and crows away. For this, the blue ribbon can only be achieved by visiting 2500 of your friends. This ribbon took me at least 6 months to complete! The same is true for fertilizing your neighbors' crops. So, you want to play FarmVille? Make sure you have neighbors.