Sorry for mentioning this again so soon, but the speed that this is ramping up is increasing. A great success story for Sony (SNE) to have after a few bad years. At this rate it will soon be one of the biggest games in the world.
Free Realms is a free to play PC MMORPG (soon to be coming to PS3). The business model involves paid for enhanced subscriptions, micropayments for in games items and the sale of playing cards for the associated physical version of the game.
The social networking side of the game is very strong. Players each have their own social networking page where they can share images and videos, send links and chat with friends. Exactly what an MMO should have.
A brilliant feature is that the game has built in video capture and videos created this way can be posted straight onto YouTube. This is going to be massive and videos of this game in action are going to be all over the internet.