The rise of social media platforms as powerful channels for corporate accountability has fundamentally altered the dynamics of corporate social responsibility (CSR) communication and its effects on consumer attitudes and behavior. Consumers — particularly Millennials and Generation Z — increasingly demand authentic, transparent, and impactful CSR commitments from brands, with the perceived authenticity of CSR communication emerging as a critical determinant of whether CSR activities generate trust, loyalty, or skepticism. This study investigates the relationships among CSR communication quality, CSR authenticity perception, consumer trust, brand loyalty, and the moderating role of social media transparency across a multi-industry sample of 524 Millennial and Generation Z consumers in the United States, United Kingdom, and Australia. A moderated-mediation model was tested using partial least squares structural equation modeling (PLS-SEM). Results demonstrated that CSR communication quality positively influenced consumer trust (β = 0.387, p < 0.001), with CSR authenticity perception fully mediating this relationship (indirect β = 0.341, 95% CI [0.268, 0.414]). Consumer trust strongly predicted brand loyalty (β = 0.523, p < 0.001). Social media transparency significantly and positively moderated the CSR communication quality–CSR authenticity perception relationship (β = 0.296, p < 0.001), such that brands perceived as highly transparent on social media derived substantially greater authenticity perceptions from equivalent CSR communication investments. The model explained 68.3% of variance in brand loyalty (R² = 0.683). Industry moderation analysis revealed the strongest CSR–loyalty effects in the fashion/apparel and food/beverage sectors, consistent with the particularly high consumer scrutiny directed at these industries. "CSR-washing" (high communication, low actual impact) was identified as significantly damaging to consumer trust (Δβ = −0.284 relative to authentic CSR), with particularly acute negative effects among Generation Z consumers. These findings provide significant theoretical and practical contributions to the CSR, branding, and consumer behavior literatures, with direct implications for corporate communication strategy in the social media age.