Social media and other digital tools have increasingly affected urban planning. Particularly, social networking sites have become new platforms of communication between governments, experts, and citizens. However, previous studies are mainly case-specific and has not yet addressed the supporting functions of social media. This study conducts a systematic review on how social media can play a role in supporting collaborative planning. Social media are combined with other digital tools to become a new form of planing support systems. They are also used directly by governments to distribute information and get feedbacks of urban plans, and by citizens to establish social networks and organize collective activisms. This study identifies several supporting functions of social media, including information distribution, social networking, public participation, and communication. However, there are also a number of issues regarding the use of social media such as a digital divide, population and spatial bias, and privacy issues. The integration of social media, other digitial tools, and traditional methods is necessary to support collaborative processes.