In this paper we investigate the activity of 1 million users tweeting under 455 different hashtags related to a wide range of topics (political activism, health, technology, sports, Twitter idioms). We find that 70% of users in the sample tweet across multiple information streams, frequently engaging in what could be described as serial activism. We furthermore determined the dominant language in each hashtag to trace which users overlap between the thematic and linguistic communities delineated by different information streams. Although social media is frequently assumed to bring together people of different nationalities and cultures to discuss a wide range of controversial issues, our results indicate that the underlying social network that connects hashtags through overlapping users is heavily limited to linguistic and content oriented communities. Information streams are clustered around linguistic communities, and hashtags within the same language group are clustered around well defined topics, such as health, entertainment and politics. The only information streams that transcend language barriers are activism related hashtags, which cluster information streams in different languages. Contrasting with the assumption that social media acts as the enabler of a globalized public debate, our results indicate a linear relationship between users who are very active in political hashtags and users who tweet across multiple political hashtags. The results suggest that activist campaigns based on social media are driven by a relatively small number of highly active, politically engaged users.