More than 2.4 billion people use the internet worldwide - including search engines and social media websites. Most of these services are free of charge - on the surface at least. But in reality, users very often "pay" with their personal data.
Personal data is like an online currency - the more precise the information, the more valuable it becomes. In his book "Fake It! Your Guide to Digital Self-Defense", San Francisco-based author and journalist Steffan Heuer discusses how internet users can seize the initiative and stop the advertising industry stealing their data.