Sociological studies on transnational migration are often based on surveys or interviews, an expensive and time consuming approach. On the other hand, the pervasiveness of mobile phones and location aware social networks has introduced new ways to understand human mobility patterns at a national or global scale. In this work, we leverage geo located information obtained from Twitter as to understand transnational migration patterns between two border cities (San Diego, USA and Tijuana, Mexico). We obtained 10.9 million geo located tweets from December 2013 to January 2015. Our method infers human mobility by inspecting tweet submissions and user's home locations. Our results depict a trans national community structure that exhibits the formation of a functional metropolitan area that physically transcends international borders. These results show the potential for re analysing sociology phenomena from a technology based empirical perspective.