In game purchases are the primary income stream for that kind of game. Advertising is a thing too – last quarter [Zynga reported 18% of revenue in advertising](https://investor.zynga.com/financial-information/quarterly-results) – but it’s not what the business is built on.
It’s worth remembering that a large portion of the income in this kind of in game purchase comes from a small portion of the player base, which is often [directly targeted with addictive mechanisms](https://www.youtube.com/watch?v=7S-DGTBZU14).
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