I spent a number of years of my youth in Las Vegas and I think that between this 'Casino' comment and potalicious' comment you've captured a lot what looks like the core issue to me.
Slot machines (especially electronic ones) are fascinating in their ability to have their payout 'tuned'. And slots typically pay out 98 - 99% of the money they take in. But what is also true is that there is a sharp 'knee' in the curve between where people sit and play slot machines for hours, and where they leave immediately after they lose their money.
The weird not immediately obvious thing, is that slots that let you 'win' a lot encourage play. When someone is sitting there and 'winning' and up 50% on their night, and then lose back to being 50% down will keep playing to 'get back' to that winning state. But people who just win enough to slow their exhaustion of cash stop when they run out. They never had a time when they felt like they were 'ahead' they just watched their cash get smaller and smaller.
New Casino owners who would get scared about big slot payouts would worry, "What if everyone takes their winnings and just leaves?" which is a legitimate worry, but you have to believe the statistics are legit and the payout is 98%, not 100+%. As it turns out those winners brag which brings in more people. And as Steve Wynn once said, "Slots make money on the quantity of the players not the quality." Meaning that the more money that goes through them the more his cut of the output.
Zynga appears focused early on revenue generation per player but not as function of all players it seemed, rather as a function of single players. By tuning the production of individuals through gameplay tweaks the over all experience is compromised such that they recognize they are being exploited and that takes away the 'fun' part of playing. At some level everyone knows they are funding Zynga (or a Casino) but they do it willingly because its 'fun'. A very fine line be 'fun' and 'not fun' when the value goes down.
If Zynga could recruit Pichette away from Google it would probably help their bottom line tremendously.