Anyone who has been on facebook longer than three seconds has gotten a Farmville, Frontierville, or something-else-ville invitation, or request to give your friend some random piece of trash to help them build something in their game. Usually these games are fairly poorly designed, but the quick rewards you can get in some cases, and forced competition make it very addictive, especially for social media users who may not be serious gamers.
For those of you who haven't played these sorts of games I will briefly explain them. These games all have a solo portion, where you use energy, or some other resource that means the same thing, to perform actions in your city/farm/whatever. You can build stuff, plant stuff, do a lot of different actions within your little world. You can also "visit" your neighbors (essentially viewing their single-player city or whatever) to help them out (doing actions that either they cannot, like reviving dead crops, or actions they can without an energy cost to them). Finally, you will reach a point where you have to build or buy something to expand your city/farm/whatever that will require either random loot (most of which you will not have) or a certain number of friends to agree to help out. So you have to spam your friends with requests until you get the required number of items or people to agree. Most of the time you can use an item-store like mechanic which uses a second in-game currency (which differs from game to game but seems present in all of them) to buy the requirements. You typically get one unit of this currency per level, but you can always buy a lot more with real cash.
The net result of this is that your friends get constant spam for stuff to help you play your game. People also usually befriend people they do not know that happen to play that same game, usually en masse. I'm not aware of anyone who just buys a massive amount of in-game currency, but I know they exist because most retailers sell either generic facebook cash cards or specific ones (for farmville and frontierville for sure). It also has the result of these not really feeling like games but political ads. I don't want to have to recruit people I don't know (and giving them access to my profile information) just to get ahead in a game. I'm also against using real money to get advantages in games.
The way these games spread, however, is through those requests. You send them to friends who don't play, and if they like the idea of the game, or just want to help you, they register, and start the vicious cycle over again. These games spread by referral and make money by their cash shop (and advertising of course). Because of this they are like a social disease, but one I wouldn't mind if they were of a higher quality.
Someone needs to develop a facebook game that doesn't require you to recruit friends to advance, and where real money gives only cosmetic or minor changes. The problem is that this ignores the advantages of social network games, in that, if you recruit others to help you play the game, the game spreads and makes its developers richer.
Just some thoughts.