This work examines how social media users learn language alongside their progression of becoming active participants in online social spaces. Using an ecolinguistic view, it explores how the process of posting and commenting on Reddit (one of the world's most popular social sites) is both afforded and constrained by the unique sociocultural histories of online spaces, previous interactions with users in those spaces, and personal values. The results, in turn, also help to provide clear illustrations of ecolinguistic principles. Thereafter, it addresses how that process of participation can result in language learning by using examples drawn from the public data available on Reddit. The participant pool of this work is comprised of 21 non-native English speakers who visited and contributed to Reddit while learning English. Their posting habits, their contributions, online participation statistics, and open-ended interviews make up the data for this project. Qualitative analyses demonstrate how users feel they learn language and highlight the position that community belonging relates to and encourages online participation. Users initiate their participation on Reddit by interacting with others in peripheral roles. Once they begin contributing and voting in specific communities, new actions are afforded. Using previous posts, comments, and experiences as co-authors, the users begin to modify the language in their contributions to both adhere to the sociocultural norms in the space and to effect predicted outcomes. This work looks deeper at how these modifications can then result in the agentic development of learners. The online behaviors of one focal participant are highlighted to provide specific examples of modified linguistic behaviors. Fine-grained cognitive event analyses reveal several measurable developments in language. However, the kind of change and the language learned which social media users experience is not readily assessed by standard measures of language acquisition, such as operationalizations of constructs like complexity or accuracy. This work applies both existing and new methods to help explain and measure the complex system of systems that make up language in online spaces. The results of this study add to a growing body of evidence supporting claims made by ecolinguists in the field of second language learning. In addition, a new concept of agentic development is highlighted and defined. Learners who participate in activities that afford agentic development are shown to modify their real-world behaviors while, at the same time, learning language. Examples of language learning observed also demonstrate how new wordings are able to, in turn, afford new actions in a cycle of perception and action. The work finishes with a brief discussion aimed at helping practitioners apply some of the principles found in this project to encourage students in their agentic development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]