Sorry, but I think you're pulling that out of your ass. All (good) games are addictive to some degree, precisely because they're fun, and I bet if you surveyed all the people who play Farmville, virtually all of them would say they enjoy it and are having fun.
Don't confuse "I don't enjoy it" with "No one enjoys it."
http://www.cracked.com/article_18461_5-creepy-ways-video-gam... + http://www.gamasutra.com/view/feature/3085/behavioral_game_d...
The techniques that I'll discuss in this article generally fall under the heading of behavioral psychology. Best known for the work done on animals in the field, behavioral psychology focuses on experiments and observable actions. One hallmark of behavioral research is that most of the major experimental discoveries are species-independent and can be found in anything from birds to fish to humans. What behavioral psychologists look for (and what will be our focus here) are general "rules" for learning and for how minds respond to their environment.
I played farmville for about a month because some friends were, by the end of that I felt like I was wasting my time, the progression was for the sake of progression, the achievements were arbitrarily chosen to take more and more time and being better than your friends had a lot more to do with time invested then any form of strategy used. I had friends who admitted they were sick of the game and it frustrated them but continued playing just to finish off achievements.
Sure I'm not speaking for everyone but I do think it is a trend in Zynga games, they are build around user engagement and seek to maximise that for as long as possible.
When I'm actually playing a game myself, though, and recognizing that these two aren't necessarily mutually exclusive, I prefer fun over addictive.
http://www.ted.com/talks/jane_mcgonigal_gaming_can_make_a_be...