That's an odd way of looking at it. To me, this is isn't even close to any sort of moral dilemma. You want to make a game and you want to make money. So you build a game thats fun, viral, and keeps people coming back. Last I recall, that's called making something people want and serving the market. There's no possible way to entrap or addict users beyond most of their free will. If they love your game so much that they keep coming back, what exactly is the problem? People WANT to have things to come back to, because there is inherent JOY in doing it.
Furthermore, building hits that can bring people back is not an easy task. It looks easy, but companies like Zynga have mastered the process and dominated the industry. You can deduce the business process of anything into a few simple sentences, yet it grossly underestimates the task at hand.
To take that view to the extreme: say I invented a new synthetic drug that was addictive. People love it because it is servicing a need in the market, but do I not have any responsibility at all for its potential side effects?
Sure, you could take it there. But the reality of it is games do not have physiologically addictive properties the same way drugs and alcohol do. If you look at the cases where gaming addiction severely impacted a person's life, you would be looking at a few outliers who probably had contributing mental factors (like most addiction problems).
We aren't talking about drugs though. We're talking about social games. If there are stories about people's lives being ruined by Farmville, I haven't heard them.
But in your example, yes, you would have a responsibility for the drug's side effects. Thats what the FDA is largely responsible for.
Furthermore, building hits that can bring people back is not an easy task. It looks easy, but companies like Zynga have mastered the process and dominated the industry. You can deduce the business process of anything into a few simple sentences, yet it grossly underestimates the task at hand.