This paper explores the suitability of social media networks (SMNs) as a means of influencing the public’s decision-making process regarding vaccinations, specifically a vaccination to protect girls against HPV, a virus associated with cervical cancer. Parents of girls in the target cohort were invited to online discussion forums where they could discuss their opinions on the vaccination. We varied the posts on the forums in different experimental condition, such that they were exposed to promotion of the vaccination in one of four different ways, and coming from one of two different sources, i.e., peers or government health representatives. Following the health belief model (HBM), these messages served as cues to action. After their active participation on the forums, participants filled out a ques-tionnaire with items related to the HBM. Analyses revealed no effect of our experimental manipula-tions of the cue to action. However, using an exploratory novel network analysis approach, we find that the HBM does not adequately account for influence via SMNs. Specifically we show that vaccination decisions are not taken in social isolation, a fact thus far ignored by various forms of the HBM. Implications for studies assessing the use of online channels for health communication are discussed.