A mobile social network system (MSNS) allows groups of friends to be accessed and engaged with from one’s mobile phone. Dodgeball is a MSNS that seeks to facilitate social connection and coordination among friends in urban public spaces. Based on a year-long qualitative field study, this article reports on the social and behavioral norms of Dodgeball use. A comparison between social network sites and Dodgeball highlights some of the communicative differences of mobile technology and the Internet. The findings of the study suggest that Dodgeball use can influence the way that informants experience public space and social relations therein. At times Dodgeball can facilitate the creation of third spaces, which are dynamic and itinerant forms of “third places.” Additionally, exchanging messages through Dodgeball can lead to social molecularization, whereby active Dodgeball members experience and move through the city in a collective manner.