The notion of audience labour has been an important contribution to Marxist political economy of the media. It revised the traditional political economy analysis, which focused on media ownership, by suggesting that media was also a site of production, constituting particular relations of production. Such analysis highlighted the active role of audience in the creation of media value as both commodities and workers, thus pointing to audience exploitation. Recently, in light of paradigmatic transformations in the media environment - particularly the emergence of Web 2.0 and social network sites - there has been a renewed interest in such analysis, and a reexamination of audience exploitation. Focusing on Facebook as a case-study, this article examines audience labour on social network sites along two Marxist themes - exploitation and alienation. It argues for a historical shift in the link between exploitation and alienation of audience labour, concurrent with the shift from mass media to social media. In the mass media, the capacity for exploitation of audience labour was quite limited while the alienation that such work created was high. In contrast, social media allows for the expansion and intensification of exploitation. Simultaneously, audience labour on social media - because it involves communication and sociability - also ameliorates alienation by allowing self-expression, authenticity, and relations with others. Moreover, the article argues that the political economy of social network sites is founded on a dialectical link between exploitation and alienation: in order to be de-alienated, Facebook users must communicate and socialize, thus exacerbating their exploitation. And vice-versa, in order for Facebook to exploit the work of its users, it must contribute to their de-alienation.