In the last two decades, Emoji have become a mainstay of digital communication, allowing ordinary people to convey ideas, concepts, and emotions with just a few Unicode characters. While emoji are most often used to supplement text in digital communication, they comprise a powerful and expressive vocabulary in their own right. In this paper, we study the affordances of �emoji-first� communication, in which sequences of emoji are used to describe concepts without textual accompaniment. To investigate the properties of emoji-first communication, we built and released Opico, a social media mobile app that allows users to create reactions - sequences of between one and five emoji - and share them with a network of friends. We then leveraged Opico to collect a repository of more than 3700 emoji reactions from more than 1000 registered users, each tied to one of 2441 physical places. We describe the design and architecture of the Opico app, present a qualitative and quantitative analysis of Opico's reaction dataset, and discuss the implications of Emoji-first communication for future social platforms.