People are still MINTING money on Facebook. I imagine the top 5 grossing facebook games make the top 5 grossing iPhone games look like adorable little hobbies. The #40 game on Facebook has 6M monthly active uniques. THe #120 game on Facebook has 2M MAU. How many iPhone games have those kind of numbers?
Platform quibbles aside, I imagine it's the fastest/cheapest way to get tens/hundreds of thousands of users to test their gameplay. It's easier to develop for Facebook than mobile, doesn't have the approval nonsense for rapid iteration, has a built in viral channel, etc.
I think it's a smart move.