Despite evidence of the acculturation benefits of social media engagement with mainstream culture, there is limited understanding of what motivates or demotivates ethnic minorities' social media engagement with mainstream culture. Adopting the theoretical construct of investment, this study interviewed 31 ethnic minority secondary school students in Hong Kong to examine their engagement with mainstream culture via everyday social media use and their concomitant investment in learning the language of the mainstream culture. Interview responses revealed that perceived ideologies of social media and the living environments, validation of and expectations regarding linguistic, cultural and social capital, and the representation and construction of desired identities were the motivational forces behind the ethnic minorities' multilingual social media engagement with mainstream culture. Multilingual social media engagement with mainstream culture was associated with changes in acculturation expectations, attitudes and resources, which motivated or constrained the ethnic minority students' investment in Chinese language learning and use on social media and in daily life. Multilingual social media use also reshaped their perceived ideologies, capital and identities. The findings call for a dialectic approach to understanding the motivational mechanisms behind ethnic minorities' use of social media, and for classroom interventions to turn the interactions into a positive cycle.