Social media platforms allow individuals without great technical knowledge or financial capital to spread violent content and radical messages among a large audience, bypassing traditional media. As violence and radical actions cause significant economic damage and undermine democratic institutions, security agencies have responded by using the same platforms to target at-risk individuals with their prevention programs. The literature has shown that such online programs can be effective in reducing victimization and perpetration rates. However, despite the large number of programs implemented, there is a significant lack of rigorous impact evaluations. This thesis contributes to the literature by developing a conceptual framework for evaluating the impact and economic efficiency of online programs for crime prevention in general. Furthermore, it contributes by evaluating the impact of two online programs in the areas of violence prevention and deradicalization. To reassess the impact of such online programs in a different prevention context, the thesis also evaluates a measure for investor protection in the context of the Bitcoin cryptocurrency.