Focusing on the long term and building a great product that is scalable ultimately leads to a great company.
Accessibility is key. MMOs are great for traditional gamers who have a lot of time to become immersed in a world, but the mass market wants things bite-sized.
First, you must have entrepreneurs who fully grasp their customers. They understand their product so well because they themselves are customers. The second thing is having an awesome value proposition.
I think many of the companies that have tried to create social worlds have failed because they approached it strictly from an opportunistic perspective.