I've never played FarmVille, but one of the things I loved about Kingdom of Loathing (other than the general awesomeness) was that I could only do so much at a time - I didn't feel like I had to commit to it if I just had a few minutes. And the fact that each day you got more adventures, it really encouraged me to keep coming back.

Oddly, it was at the point where I had more adventures in a day than I had time to use (and therefore started loosing them) was the point that I started to loose interest.