MicroJam is a mobile app for sharing tiny touch-screen performances. Mobile applications that streamline creativity and social interaction have enabled a very broad audience to develop their own creative practices. While these apps have been very successful in visual arts (particularly photography), the idea of social music-making has not had such a broad impact. MicroJam includes several novel performance concepts intended to engage the casual music maker and inspired by current trends in social creativity support tools. Touch-screen performances are limited to five seconds, instrument settings are posed as sonic ``filters'', and past performances are arranged as a timeline with replies and layers. These features of MicroJam encourage users not only to perform music more frequently, but to engage with others in impromptu ensemble music making.