Frickin numbers.
I remember EA, etc, looking at Facebook gaming about a year ago or so, and saying, "This is the future."
This was a frightening moment.
Though, really, it makes sense.
As social gaming, in MMOs, and the casual market becomes more prominent, Facebook, already an addiction for a ridiculous number of people, becomes the ideal market.
Particularly when you have a number of different currencies to convert through, turning real cash into less-scary-to-spend 'tokens.'
It's amazing how adept humans have become at finding new, excellent ways to squeeze money out of other humans.