The diversity of agendas in the media plays a critical role in shaping the scope of public discourse. Due to the increasing impact of social media, it is important and necessary to examine if it is circulating a wide range of issue and extending the realm of public discussion. In this paper, we put forward agenda diversity as a core value for evaluation of social media discourse and examine the current diversity status. Our study provides a firm basis for understanding the topics, quantity, diversity, and relative salience of the agendas offered by the social media. We further look into the information diffusion process from the diversity viewpoint, and explore the potential to increase diversity.