Hm. I'm not talking scientifically here or anything, but I've always felt the lack of any kind of depth in most Zynga games makes them feel very skinner box with little pretense beyond that. Most games have complicated stories or game play mechanics. Many Zynga games seem to be "Fill up these three bars by clicking these buttons. Watch them fill. It feels good doesn't it."

Personally I couldn't say that about many other games/work.