The theory goes like this, if you make an element of the gameplay involve other people, such as leaving a gift for someone in Farmville, then you're more likely to return and play more due to the social obligation. Another framework is layered below that, which is to have everything take a specified amount of time to complete. Farming crop X takes 2 hours and crop Y takes 4 hours. Two of ngmoco's freemium games use these dynamics, but they follow two models. As a player, one is clearly superior in my mind (We Rule), but as a business owner, I think they got it backwards.
Note: this blog entry doesn't address the use of 'for pay' instant gratification gameplay. Each game uses it in the same way and isn't the interesting story (imho).
Both games leverage a social obligation and a decay of 'stuff' in the game, to encourage you to login and do some stuff. That of course increases the amount of time you spend looking at the game which generates ad impressions. In terms of the ad supported business model, I think GodFinger should be an ad supported game, just like We Rule. Here's why...
In We Rule, I have many options for the types of crops to plant, each with its own time to maturity. Those time periods range from 5 minutes to a full day (or longer). If I know I don't want to, or can't, play for a set amount of time, I just plant a long term crop and then logoff. If someone places an order at one of my businesses, I can let it sit there for a long time before approving it. So, the bottom line is that I can control the amount of time I put into the game and how often I play it. The incentive to come back and login RIGHT NOW isn't very strong since I scheduled my crop maturity time period. Any orders placed at my business can be safely ignored since those business will generate money anyway, albeit more slowly.
GodFinger on the other hand is NOT ad supported but has a "you should come back and play right now" model that I CAN'T control. If I want to optimize my cash flow in the game, I have to log back in much more regularly to deal with things. Since, I have to log back in when the game wants me to, it would be the better game to use an ad supported model, but for some reason, it doesn't have ads and only wants me to spend money on something the game gives me anyway (slowly).