Integrating social media into daily life has revolutionized marketing communications, making them more interactive and multimodal, yet understanding of its impact on business-to-consumer (B2C) and consumer-to-consumer (C2C) interactions remains limited. This research investigates B2C and C2C communications in street fashion brand advertising on Instagram (IG) from a cross-cultural perspective, exploring the influence of multimodality, interactivity, and group polarization on consumer behavior. Employing a mixed-methods approach, it reveals differences in content presentation by Asian and Western brands and the effects of group dynamics on viewer attitudes, offering insights for enhancing social media marketing strategies and contributing to consumer behavior analysis.