Recent evidence shows that social media use has negative effects on well-being of children and youths. However, the underlying reasons are unclear, as social media are means that can also serve beneficial purposes. We propose the hypothesis that social media induce users to harmful addiction of a new variety because such use is not toxic per se, but becomes toxic by crowding out beneficial activities. We identify, in particular, the key mechanism in the change of time preference: while social media induce users to present-biased activities, thus encouraged by how platforms are designed, they crowd out activities that develop skills and are forward-looking, such as education, volunteering and democratic participation. This triggers a vicious circle leading to a long run deterioration of well-being and skills that would have acted as an antidote to addiction. As implication, policies should address adequate information and education in general, as well as increased competition in the digital platform market. While the available evidence supports our hypothesis in many respects, more empirical research is needed.