The graph is from the new paper "Geographic Dissection of the Twitter Network" Juhi Kulshrestha et al (2012). It concludes:
In this paper, we attempted to address the question: does of- fline geography still matter in online social networks? To this end, we dissected the Twitter social network based on users’ geolocations and investigated how users’ geoloca- tions impact their participation in Twitter, their connectivity with other users, and the information they exchange with them. Our in-depth analysis reveals that geography crucially impacts all aspects of the Twitter social network. Specifically, we find that even though users preferentially con- nect and exchange information with other users from their own country, more than a third of all links and tweets are exchanged across national boundaries. Such transnational links and interactions occur between users in geographically and linguistically proximal countries. Our findings have potential applications in predicting or recommending social connections for a user as well as in understanding global diffusion of information.