If users are not doing what the designer intended (when users are investing time, effort, etc in your product), the designer may be asking them to do too much.

To initiate (user) action, doing must be easier than thinking.

You'll often find that people's declared preferences - what they say they want - are far different from their revealed preferences - what they actually do.

When designers intentionally trick users into inviting friends or blasting a message to their social networks, they may see some initial growth, but it comes at the expense of users' goodwill and trust. When people discover they've been duped, they vent their frustration and stop using the product.

As fleeting commitments, diets often fail. Thinking of dietary choices as part of who you are...can give them real staying power

For an infrequent action to become a habit, the user must perceive a high degree of utility, either from gaining pleasure or avoiding pain.

The aim is to influence customers to use your product on their own, again and again without relying on overt calls to action such as ads or promotions.