Citizens are expected to make informed voting decisions. However, research indicates that political knowledge gained through media use does not relate to political participation such as voting. In times of decreasing voter turnout and increasing consumption of news via social media, it is important to study how these two relate. Recent research underscores the fact that, in reality, social media does not increase objective political knowledge, but rather the metacognition of subjective knowledge. In turn, this metacognition might foster political participation. Nevertheless, we do not know which forms of social media use foster users’ perception of being politically knowledgeable. A pre-registered, cross-sectional, pre-election survey (N = 1,223) showed that active forms of social media news use relate more strongly to subjective knowledge than incidental exposure. All forms of usage showed no or even negative associations with objective political knowledge. While none of the forms of social media news use exerted any direct effect on voting intentions, both subjective and objective knowledge are related to increased voting intentions. This study corroborates that social media does not affect political participation directly, but rather through metacognitive processes such as estimating one’s knowledge. However, both objective and subjective knowledge are essential for one’s voting intentions. By showing that active and passive forms of social media use affect knowledge differently, this study provides preliminary and nuanced insights into the ultimate role these technologies can play in democratic processes.