Abstract: This paper examines negative phenomena arising from algorithmic technology in platform communication, including information cocoons, trending topic manipulation, infringement of labor rights, and big-data price discrimination against existing customers. It argues that these seemingly distinct negative phenomena collectively point to a deep-seated structural issue: the crisis of publicness in platform communication. The occurrence mechanisms of this crisis are manifested as follows: the commodification of public issues, the fragmentation of public dialogue, algorithmic substitution of norms for public communication, and algorithmic black-boxing alongside the failure of public oversight. On this basis, guided by the value of digital justice, the paper proposes governance pathways including: the pluralistic reconstruction of public agenda-setting, algorithmic repair of public dialogue space, institutional optimization of norms for public communication, and algorithmic adaptation of public oversight mechanisms.