Using nationally representative data from the 2020 and 2024 American National Election Studies (ANES), this paper traces how the U.S. social media landscape has shifted across platforms, demographics, and politics. Overall platform use has declined, with the youngest and oldest Americans increasingly abstaining from social media altogether. Facebook, YouTube, and Twitter/X have lost ground, while TikTok and Reddit have grown modestly, reflecting a more fragmented digital public sphere. Platform audiences have aged and become slightly more educated and diverse. Politically, most platforms have moved toward Republican users while remaining, on balance, Democratic-leaning. Twitter/X has experienced the sharpest shift: posting has flipped nearly 50 percentage points from Democrats to Republicans. Across platforms, political posting remains tightly linked to affective polarization, as the most partisan users are also the most active. As casual users disengage and polarized partisans remain vocal, the online public sphere grows smaller, sharper, and more ideologically extreme.

翻译：本文基于2020年和2024年美国国家选举研究（ANES）的全国代表性数据，追踪了美国社交媒体格局在平台、人口结构和政治倾向维度的演变。总体平台使用率呈下降趋势，最年轻和最年长的美国人正逐渐完全脱离社交媒体。Facebook、YouTube和Twitter/X的用户基础萎缩，而TikTok和Reddit实现小幅增长，反映出数字公共领域日趋碎片化。各平台用户群体呈现老龄化特征，教育水平和多样性略有提升。政治倾向上，多数平台向共和党用户偏移，但整体仍保持民主党倾向。Twitter/X的转变最为显著：发帖用户中民主党与共和党比例发生近50个百分点的逆转。跨平台数据显示，政治性发帖行为与情感极化高度相关，最具党派倾向的用户也最为活跃。随着普通用户逐渐退出而极化群体持续发声，网络公共领域正变得规模更小、立场更尖锐、意识形态更趋极端。