This study examines how a Terms of Service update on X enabling default AI training on user content activated privacy anxiety and reshaped user behavior. Privacy anxiety is conceptualized as a structural outcome of reduced control over data use, particularly among content creators. The study finds that privacy anxiety is activated within creator communities and diffused across user groups through inter- and cross- community interaction. As anxiety escalated, engagement declined and migration intentions increased. These findings point to an unresolved dilemma in AI-driven platform governance: how user trust and autonomy can be sustained under conditions of concentrated power and data-dependent business models remains unclear.