Social media platforms mediate professional communication, political expression, and community formation, making the rare instances when users collectively abandon an incumbent platform particularly consequential. Strong network effects raise switching costs and strengthen incumbents' positions, making coordinated exit difficult. Here we link 276,431 scholars on Twitter/X to their respective new profiles among the universe of all 16.7 million Bluesky accounts, tracked from January 2023 to December 2024, using a scalable, high-precision cross-platform matching pipeline. Exploiting exogenous variation from Brazil's court-ordered suspension of Twitter/X and a dynamic matching design, we show that adoption is peer-driven, treatment effects are short-lived and dose-dependent, and contagion is simple, not complex. Three patterns characterize adoption and retention. Adoption concentrates among users deeply embedded in Twitter's social graph. Public political expression predicts migration, consistent with homophilous inflows into a largely left-of-center Bluesky information space. Early reconnection with prior contacts predicts longer tenure and engagement. Our findings provide the first population-scale causal evidence of peer influence in a social media platform migration by exploiting exogenous exposure variation in a natural experiment and using daily dynamic matching. Rather than the complex contagion mechanism often emphasized in the literature, contagion is predominantly simple. Our findings recast migration as a multi-homing strategy that insures against governance uncertainty and show that users who quickly reconnect with prior contacts remain active longer on Bluesky.

翻译：社交媒体平台作为专业交流、政治表达和社区构建的中介，使得用户集体放弃现有平台这一罕见现象具有重大影响。强大的网络效应提高了转换成本并巩固了现有平台的地位，使得协调退出变得困难。本研究通过可扩展、高精度的跨平台匹配流程，将276,431名Twitter/X学者用户与全部1,670万Bluesky账户中的新档案进行关联追踪，时间跨度为2023年1月至2024年12月。利用巴西法院强制暂停Twitter/X服务所产生的外生变异及动态匹配设计，我们发现：平台采用具有同伴驱动性，处理效应呈现短期性和剂量依赖性，且传染机制属于简单传染而非复杂传染。采用与留存呈现三个特征模式：采用行为集中于深度嵌入Twitter社交图谱的用户；公开政治表达可预测迁移行为，这与左倾主导的Bluesky信息空间中的同质化流入现象一致；早期与原有联系人重建联系能预测更长的使用周期和参与度。本研究通过自然实验中的外生暴露变异和每日动态匹配，首次为社交媒体平台迁移中的同伴影响提供了人口规模的因果证据。与文献中常强调的复杂传染机制不同，传染过程主要表现为简单传染。研究结果将平台迁移重新诠释为应对治理不确定性的多归属策略，并表明快速重建社交联系的用户在Bluesky上保持更长的活跃周期。