I'm pleased for these guys, but a little surprised as well. Launching a Pokemon social game isn't exactly being disruptive; it's not as if there's any dearth of them on Facebook, particularly at a time when many companies are fleeing the already strip-mined platform for greener pastures.
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.
That isn't even necessary as a success condition, but in general, YC invests in Texas Hold'em because they see the glimmer that suggests you're capable of eventually executing on FarmVille. (Zynga wasn't Zynga on day one, either.)
