Discovering high-level topics from social streams is important for many downstream applications. However, traditional text mining methods that rely on the bag-of-words model are insufficient to uncover the rich semantics and temporal aspects of topics in Twitter. In particular, topics in Twitter are inherently dynamic and often focus on specific entities, such as people or organizations. In this paper, we therefore propose a method for mining multifaceted topics from Twitter streams. The Multi-Faceted Topic Model (MfTM) is proposed to jointly model latent semantics among terms and entities and captures the temporal characteristics of each topic. We develop an efficient online inference method for MfTM, which enables our model to be applied to large-scale and streaming data. Our experimental evaluation shows the effectiveness and efficiency of our model compared with state-of-the-art baselines. We further demonstrate the effectiveness of our framework in the context of tweet clustering. More info: http://www.cse.ust.hk/~jvosecky/