Prior research has documented cultural dimensions that broadly characterize between-culture variations in Western and East Asian societies and that bicultural individuals can flexibly change their behaviors in response to different cultural contexts. In this article, we studied cultural differences and behavioral switching in the context of the fast emerging, naturally occurring online social networking, using both self-report measures and content analyses of online activities on two highly popular platforms, Facebook and Renren (the "Facebook of China"). Results showed that while Renren and Facebook are two technically similar platforms, the Renren culture is perceived as more collectivistic than the Facebook culture. Furthermore, we presented evidence for the first time that users who are members of both online cultures flexibly switch and adapt their in-group sharing behaviors in response to the online community in which they are: They perform more benevolent in-group sharing when they participate in the Renren community and less so when they participate in the Facebook community. We discussed both the theoretical and methodological implications of the current research.