As platforms increasingly scale down professional fact-checking, community-based alternatives are promoted as more transparent and democratic. The main substitute being proposed is community-based contextualization, most notably Community Notes on X, where users write annotations and collectively rate their helpfulness under a consensus-oriented algorithm. This shift raises a basic empirical question: to what extent do users' social dynamics affect the emergence of Community Notes? We address this question by characterizing participation and political behavior, using the full public release of notes and ratings (between 2021 and 2025). We show that contribution activity is highly concentrated: a small minority of users accounts for a disproportionate share of ratings. Crucially, these high-activity contributors are not neutral volunteers: they are selective in the content they engage with and substantially more politically polarized than the overall contributor population. We replicate the notes' emergence process by integrating the open-source implementation of the Community Notes consensus algorithm used in production. This enables us to conduct counterfactual simulations that modify the display status of notes by varying the pool of raters. Our results reveal that the system is structurally unstable: the emergence and visibility of notes often depend on the behavior of a few dozen highly active users, and even minor perturbations in their participation can lead to markedly different outcomes. In sum, rather than decentralizing epistemic authority, community-based fact-checking on X reconfigures it, concentrating substantial power in the hands of a small, polarized group of highly active contributors.

翻译：随着平台日益缩减专业事实核查的规模，基于社区的替代方案因其更高的透明度和民主性而受到推崇。当前提出的主要替代方案是基于社区的语境化标注，其中最引人注目的是X平台上的社区笔记功能：用户撰写注释，并在共识导向算法下共同评价其有用性。这一转变引发了一个基础性的实证问题：用户的社会动态在多大程度上影响社区笔记的生成？我们通过分析参与行为与政治倾向来探讨这一问题，使用了2021年至2025年间公开的全部笔记与评分数据。研究发现，贡献活动高度集中：少数用户承担了不成比例的评分任务。关键的是，这些高活跃度贡献者并非中立的志愿者：他们在参与内容上具有选择性，且政治极化程度显著高于整体贡献者群体。我们通过整合生产环境中使用的社区笔记共识算法的开源实现，复现了笔记的生成过程。这使得我们能够通过改变评分者群体来修改笔记的显示状态，进行反事实模拟。结果表明，该系统存在结构不稳定性：笔记的生成与可见性往往取决于几十名高活跃用户的行为，即使其参与度发生微小扰动也可能导致显著不同的结果。总之，X平台上的社区事实核查并未分散认知权威，而是对其进行了重构，将实质性权力集中于一小群高度极化且极为活跃的贡献者手中。