This study uses a cluster randomized controlled trial to evaluate the impact of a nationwide malaria prevention advertising campaign delivered through social media in India. Ads were randomly assigned at the district level, and the study relies on data from two independently recruited samples (8,257 individuals) and administrative records. Among users residing in solid (concrete) dwellings, where malaria risk is lower, the campaign led to an 11 percent increase in mosquito net usage and a 13 percent increase in timely treatment seeking. Self-reported malaria incidence decreased by 44 percent. Consistently, recorded health facility data indicate a reduction in urban monthly incidence of 6.2 cases per million people, corresponding to 30 percent of the overall monthly incidence rate of malaria. Conversely, the study finds no impact on households living in non-solid dwellings, which face higher malaria risk, nor among rural settlements where such dwellings are more prevalent. To disentangle if this lack of impact stems from ineffective content or insufficient reach, an individual-level trial was conducted (1,542 individuals), ensuring campaign exposure for both household types. The findings indicate an increase in bed net usage and timely treatment seeking for both groups, underscoring the need for improved targeting in social media campaigns to fulfill public health goals.