When the Tamagotchi was first released in 1996 it was a super hit, selling more than 70 million units. This virtual pet craved your attention and appealed to your inner human. It would eventually die out if the appeals were unanswered.
With technology becoming more robust, more single user games were created. Games like Lemonade Tycoon (2002) which gave you a sense of fulfillment in running your own lemonade stand, earning and spending money, making decisions, getting a defining sense of self, were a hit.
The Sims of SIM city were another story. First released in 2000, this was a simulation game in which a player could control the lives of characters, their daily activities, stimulating the inner GOD in the player. 
Come social networking, and now you have single player games, which help you to connect with others in your network, get them on board, get them hooked and still continue being master of your domain, lord of all that you survey. Farmville in particular is one that I have been playing for a few days now.
Farmville is funded by direct purchases of Farmville money or through its partner sites which would have a user participate in a survey or other such activity.
The thrill of living vicariously through a virtual character, earning coins, making sure that your crops don’t rot, is becoming addictive. There must be a subliminal message that is being passed through platform. What is the chord that they have struck that resonates through ones being, that makes them wake up in the morning, forego their coffee and visit a pixelated farm to ensure everything is fine??!!
Mentabolism 