Deplatforming, the permanent banning of entire communities, is a primary tool for content moderation on mainstream platforms. While prior research examines effects on banned communities or source platform health, the impact on alternative platforms that absorb displaced users remains understudied. We analyze four major Reddit ban waves (2015--2020) and their effects on generalist communities on Voat, asking how post-ban arrivals reshape community structure and through what mechanisms transformation occurs. Combining network analysis, toxicity detection, and dynamic reputation modeling, we identify two distinct regimes of migration impact: (1) Hostile Takeover (2015--2018), where post-ban arrival cohorts formed parallel social structures that bypassed existing community cores through sheer volume, and (2) Toxic Equilibrium (2018--2020), where the flattening of existing user hierarchy enabled newcomers to integrate into the now-dominant toxic community. Crucially, community transformation occurred through peripheral dynamics rather than hub capture: fewer than 5% of newcomers achieved central positions in most months, yet toxicity doubled. Migration structure also shaped outcomes: loosely organized communities dispersed into generalist spaces, while ideologically cohesive groups concentrated in dedicated enclaves. These findings suggest that receiving platforms face a narrow intervention window during the hostile takeover phase, after which toxic norms become self-sustaining.

翻译：平台封禁（即永久性禁止整个社区）是主流平台进行内容审核的主要手段。尽管已有研究考察了封禁对被禁社区或源平台健康度的影响，但对于接收流离用户的替代平台所受到的影响仍缺乏深入探讨。本文分析了Reddit的四次主要封禁浪潮（2015-2020年）及其对Voat综合性社区的影响，探究封禁后涌入用户如何重塑社区结构以及转变发生的机制。通过结合网络分析、毒性检测和动态声誉建模，我们识别出两种不同的迁移影响机制：（1）敌对性接管（2015-2018年）：封禁后涌入的群体通过规模效应形成平行社会结构，绕过了现有的社区核心；（2）毒性均衡（2018-2020年）：现有用户层级的扁平化使新用户得以融入当时已占据主导的毒性社区。关键发现表明，社区转型通过边缘动态而非中心节点捕获实现：在大多数月份中，仅不足5%的新用户能占据核心地位，但社区毒性却翻倍增长。迁移结构同样影响结果：松散组织的社区分散到综合性空间，而意识形态凝聚的群体则集中于专属领域。这些发现提示接收平台在敌对性接管阶段存在短暂的干预窗口期，此后毒性规范将形成自我维持的循环。