Then you're hooked.

As a consultant to Silicon Valley startups, Eyal helps his clients mimic what he calls the ‘narcotic-like properties’ of sites such as Facebook and Pinterest. His goal, Eyal told Business Insider, is to get users ‘continuing through the same basic cycle. Forever and ever.’ In Hooked, he sets out to answer a simple question: ‘How is it that these companies, producing little more than bits of code displayed on a screen, can seemingly control users’ minds?’ 

The answer, he argues, is a simple four-step design model. Think of Facebook’s news feed. The first two steps are straightforward – you encounter a trigger (whatever prompts you to scroll down on the feed) and an opportunity for action (you actually scroll down). Critically, the outcome of this action shouldn’t be predictable – instead, it should offer a variable reward, such that the user is never quite sure what she’s going to get. On Facebook, that might be a rewarding cat video, or an obnoxious post from an acquaintance. 

Finally, according to Eyal, the process should give you a chance to make some kind of investment – clicking the Like button, for example, or leaving a comment. The investment should gradually ramp up, until the user feels more and more invested in the cycle of trigger, action and reward.

 Michael Schulson, User behaviour.