Where Farmville differs from other games of this type is the way that goods and values shift by incorporating the game into Facebook’s social network. In order to upgrade the size of your farm, which allows you to plant more crops, you can either “neighbor” a certain number of people or pay real money. Neighboring someone in Farmville is a bit like friending them on Facebook in that you’re probably not going to expose yourself to a total stranger, so this usually means recruiting people from your social circle to play.
Neighbors are also handy to have because you can go visit their farm to gain experience by weeding or fertilizing their crops. This tends to help the person doing the fertilizing more than it helps the one receiving the service, so such motivation is automatic. The ultimate effect is that Farmville creates a strong incentive for players to recruit their friends, and perhaps more importantly, to actively visit each other’s farms and look around. Players cannot negatively impact each other in anyway. They can only help.
My curiosity about Farmville is almost enough to make me reactivate my Facebook account. Almost.
I focused on Harvest Moon: Save the Homeland (along with Morrowind and Okami) in my dissertation. More than once, I've read about the similarities between Harvest Moon and Farmville, thereby piquing my curiosity even more.