Not only do they add no value to society, one could argue they are responsible for decreased productivity of employees who waste their work day playing Zynga games or come back to work tired after wasting 2 hours at night planting cucumbers on FarmVille.
Can you help me understand the contempt you have for the Zynga games? I don't play, myself, but I do understand that the games exploit (negative? compulsive?) psychology, and they've used unethical business strategies.
But who cares? After all, it's unlikely the next Vonnegut is going to spend his time playing MafiaWars instead of writing "Slaughterhouse Five" or anything.
I am reminded of the aphorism, "Time you've enjoyed wasting is not wasted time."