From my understanding, its multiple buildings. They're going to be pretty freaking huge, but still multiple buildings. Iowa already has tax incentives to help attract companies like Google and Microsoft. I'm not surprised that Facebook is building there. 2 separate power grids (with lots of wind energy), no flooding, you can build buildings to withstand some pretty powerful winds [ Take a tour of LightEdge's data center right across the Interstate to see for yourselves ] not to mention all the dark fiber running all across the state.
I think its good for Iowa, but I think you hit the point where its a bit of bragging rights and inflation of the numbers to get even more tax breaks and special considerations like lower rates on water, etc. But when it comes down to it, its not going to create many long term high paying jobs which is what the politicians will tout. It will make a few average income workers that will quickly burn out when they realize that they are server monkeys just replacing a server when the light flashes red and nothing more.