1. Starbucks Gold Card
As a symbol of exclusivity and status, the Starbucks gold card does a great job of increasing engagement among an already loyal customer base. To get the card, you need to buy 30 drinks a year–a lot, but not so much that it’s prohibitive for people to obtain. The perks aren’t even really that great (free drink every 15 drinks or so), but the idea of having a gold card with your name on it makes the status very desirable.
There’s no logical reason why people would take so much care, devote so much time, and spend so much money to a virtual farm. That said, Zynga’s whole model is built upon the desirability of building a virtual empire and taking care of imaginary animals. They’ve done incredibly well so far doing that, and all of their subsequent games have also been built on the idea that people will pay to upgrade a virtual world.