Applying motivated reasoning theory, this study explores how people pay attention to politicized science issues and engage in supportive actions in social media. In doing so, the role of issue confidence was closely investigated. The current study adopts a non-profit organization’s use of political ideology and hoaxes in promoting labeling issues for GMO (genetically modified organisms) products. Results from an online survey with an embedded experiment showed that issue confidence is related to an individual’s motivation to engage in the GMO labeling issue, unlike factual knowledge or literacy. Findings suggested that social media hoaxes can activate individuals’ issue confidence. The present study discussed not only the effects of issue confidence and social media hoaxes but also ethical considerations of hoax-spreading by activists.