Farmville became a viral game because it relies on the charity of others. Your friends and family are given a constant play-by-play of your new purchases and achievements, which quickly drown their facebook wall with requests for help.
At the same time, in order to succeed in all areas, you’ll be required to recruit people to join the game and begin little digital farms of their own. If they refuse, you’ll need to convince them to try it, which begins the process over again. Woe to those who would uninstall the game and disappear from the friends list of the hardcore “players” out there.