<p>Online English learning communities are a very dynamic way of developing language with peers and in a collaborative way of learning. This research has been inspired by the need to explore How the social capital formed in these communities leads to better language proficiency with time. The study employed a quantitative longitudinal research design where data were obtained at 4 points in time among active participants in English learning communities in social networks (<i>N</i> = 373 at baseline; 319 at 3 months; 282 at 6 and 12 months). The results indicate that the higher the participation in these communities, the larger the amount of social capital developed, which in turn has a positive influence on language proficiency and long-term retention. The mediation role between participation and language outcomes is played by collaborative learning behaviors, and the moderation is played by platform features. The novelty of this study lies in integrating the aspects of participation, social capital, and platform features in a long-term model of language learning, providing both theory and practical value to educators and platform designers.</p>