Ideas are a dime a dozen; stealing the execution is another thing altogether.
"If you’ve played Farm Town at any point in the past few months then you won’t need any education about how to use Zynga’s version of the game as it’s almost an exact duplicate."
http://www.chinasocialgames.com/?p=400
> myFarm was the original farming game on Facebook. We launched Nov 1st 2008
It's a messy chain of copycats.
Farmville was more or less a clone of Farm Town, though it evolved at a much faster pace.