The entertainment-driven dynamics of social media platforms encourage users to engage with like-minded individuals and consume content aligned with their beliefs. These dynamics may amplify polarization by reinforcing shared perspectives and reducing exposure to diverse viewpoints. Simultaneously, users migrate from one platform to another, either forced by moderation policies, such as de-platforming, or spontaneously seeking environments more aligned with their preferences. These migrations foster the specialization and differentiation of the social media ecosystem, with platforms increasingly organized around specific user communities and shared content preferences. This shift marks an evolution from echo chambers enclosed within platforms to "echo platforms", i.e., entire platforms functioning as ideologically homogeneous niches. This study introduces an operational framework to systematically analyze these dynamics, by examining three key dimensions: platform centrality (central vs. peripheral), news consumption (reliable vs questionable), and user base composition (uniform vs diverse). To this aim, we leverage a dataset of 126M URLs posted by nearly 6M users on nine social media platforms, namely Facebook, Reddit, Twitter (now X), YouTube, BitChute, Gab, Parler, Scored, and Voat. We find a clear separation between mainstream and alt-tech platforms, with the second category being characterized by a peripheral role in the social media ecosystem, a greater prevalence of unreliable content, and a heightened ideological uniformity. These findings outline the main dimensions defining the fragmentation and polarization of the social media ecosystem.

翻译：社交媒体平台的娱乐驱动机制鼓励用户与志同道合者互动，并消费符合其信念的内容。这些机制可能通过强化共同观点并减少接触多元视角而加剧极化现象。与此同时，用户在不同平台间迁移——既可能因内容审核政策（如平台封禁）被迫迁移，也可能自发寻求更符合偏好的环境。这种迁移促进了社交媒体生态系统的专业化与差异化，使得平台日益围绕特定用户社群和共享内容偏好进行组织。这一转变标志着从平台内部的"回音室"演变为"回音平台"，即整个平台成为意识形态同质化的生态位。本研究提出一个操作性分析框架，通过考察三个关键维度系统分析这些动态：平台中心性（核心与边缘）、新闻消费（可靠与可疑）以及用户构成（同质与多元）。为此，我们利用包含约600万用户在九个社交媒体平台（即Facebook、Reddit、Twitter（现X）、YouTube、BitChute、Gab、Parler、Scored和Voat）发布的1.26亿条URL的数据集展开分析。研究发现主流平台与替代技术平台之间存在明显区隔：后者在社交媒体生态中处于边缘地位，不可靠内容盛行度更高，且意识形态同质性更强。这些发现勾勒出定义社交媒体生态系统碎片化与极化的主要维度。