Two experiments examined if persuasive effectiveness of health messages varies as a function of the communication channel (Facebook vs. news website), and if so, why. Specifically, we examined perceived ulterior motives of the communicator as an explanation for why public health campaigns may be less effective when conveyed via mass-directed (vs. interpersonal) channels, and further investigated if message recipients' health interest moderates such channel effects, if any. In Study 1 (N = 103), reading a medical news reporter's Facebook post on dental health (vs. a news article consisting of the identical content) lowered the participants' suspicion of ulterior motives of the source, which then promoted message-consistent attitudes and behavioral intention. Such effects, however, emerged only for those more interested in health. Using a different topic (a low-carb, high-fat diet), Study 2 (N = 338) replicated Study 1 findings, confirming the conditional persuasive advantages of social media over mass media as a health campaign channel.