AI-powered companion chatbots (AICCs) such as Replika are increasingly popular, offering empathetic interactions, yet their psychosocial impacts remain unclear. We examined how engaging with AICCs shaped wellbeing and how users perceived these experiences. First, we conducted a large-scale quasi-experimental study of longitudinal Reddit data, applying stratified propensity score matching and Difference-in-Differences regression. Findings revealed mixed effects -- greater grief expression and interpersonal focus, alongside increases in language about loneliness, depression, and suicidal ideation. Second, we complemented these results with 18 semi-structured interviews, which we thematically analyzed and contextualized using Knapp's relationship development model. We identified trajectories of initiation, escalation, and bonding, wherein AICCs provided emotional validation and social rehearsal but also carried risks of over-reliance and withdrawal. Triangulating across methods, we offer design implications for AI companions that scaffold healthy boundaries, support mindful engagement, support disclosure without dependency, and surface relationship stages -- maximizing psychosocial benefits while mitigating risks.

翻译：以Replika为代表的人工智能伴侣聊天机器人（AICC）日益普及，它们能提供共情式互动，但其社会心理影响尚不明确。本研究探讨了与AICC互动如何影响用户福祉，以及用户如何感知这些体验。首先，我们对Reddit平台的纵向数据进行了大规模准实验研究，采用分层倾向得分匹配与双重差分回归方法。研究结果呈现出混合效应：用户更频繁表达悲伤情绪并更关注人际关系，同时涉及孤独、抑郁及自杀意念的语言表达也有所增加。其次，我们通过18场半结构化访谈对上述结果进行补充，并运用克纳普关系发展模型进行主题分析与情境化阐释。研究发现用户与AICC的关系呈现启动、升级和绑定三个阶段的发展轨迹：AICC既能提供情感确认与社会演练机会，也存在过度依赖与社交退缩的风险。通过跨方法三角验证，我们为AI伴侣设计提出以下建议：构建健康边界、支持正念互动、实现无依赖式自我表露、显性化关系发展阶段——从而在最大化社会心理效益的同时降低潜在风险。