This research aims to address the literature gaps by investigating the associations between social media usage and investor optimism, especially when considering the interaction between subjective and objective investment knowledge. Utilizing the 2021 National Financial Capability Study (NFCS) main dataset and its supplementary Investor Survey, this study specifically examines how the use of social media as an investment information source affects investors’ portfolio performance expectations. Particular attention is paid to comparing investors who anticipate above-market performance versus those expecting market-equivalent returns, with investment knowledge serving as a potential mediating factor.