I've played these games and worked in the Addictive Game industry. In my experience, every time the addiction is wearing off, you have to 'up the ante' (either by introducing new content, new mechanics, upping the difficulty curve etc). But the problem is that the addiction is mildly stressful on the player, and every time they increase the addiction mechanics, they increase the stress.

This inevitably leads to burnout and it usually happens in one of two ways:

1-they run out of content or master the learning curve, therefore they run out of stuff to be addicted to; this usually causes a general tapering off of play time

2-the stress overcomes the addiction and they flat out quit, even though they are still addicted

It would be interesting to examine the play patters of various games that have different variations on addiction vs stress vs fun.

LoL -high fun, high stress, low addiction

WoW -med fun, med stress, med addiction

farmville -low fun, high stress, high addiction

edit: the stress of a game like LoL is very different from a game like FV. LoL is high because you must play perfectly in order not to be ridiculed by your teammates. FV is high because you are given a dozen new things to do every time you just want to do the basic stuff, but your compelled to do everything, which generally requires you to keep a large mental queue of everything it throws at you every time you log in.