I wouldn't agree with that, initial fun is important but over the longer term I think addictiveness can take over fun, to the point where the person feels obligated to continue playing the game to continue progressing even though each gaming session is in a sense almost dreaded.

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.