The educational potentials of social media both in the formal and informal learning contexts have been widely acknowledged. However, how social media use in the informal contexts might influence students' learning in the formal contexts is still underexplored. Path analysis of 141 survey responses from secondary school ethnic minority students in Hong Kong revealed that voluntary access to Chinese social media in daily life influenced these students' ideal L2 self and motivated efforts in learning Chinese both directly and indirectly via bicultural integration identity and bicultural competence. The findings confirmed that social media practices in the informal contexts may influence students' motivated efforts in learning in the formal contexts. The study suggests promoting ethnic minorities' extramural use of mainstream culture social media to influence their acculturation into the dominant culture and motivation in learning the dominant culture's language. It further highlights the importance of equipping ethnic minority students with the necessary socio-cultural and communication skills to facilitate positive intercultural engagement on these sites so as to safeguard the positive influences of informal social media use on students' motivation for learning.