Social media are some of the most used digital composition tools by both youth and adults yet authoring in digital spaces remains undervalued and digital literacy education remains misaligned with workplace needs and expectations. Using a multiple case study design (n=3) to explore the authorship of pre-service English/ELA teachers on social media and how it impacts their composition instruction, this study forefronts social media as a critical space for authoring that should be considered in the context of education. Multiple interview sessions and composition artifacts (e.g., social media posts, course assignments, creative writing) were used to gather stories of the pre-service teachers' authorship experiences and their approaches to composition instruction. This study speaks to the need to reconsider what counts as valuable literacies; pre-service teachers' frequent authoring on social media cannot be siloed away from the authorial identities they bring into their classrooms and their classroom instruction. Final analyses offer implications for future research at the intersection of authorship theory, social media, and pre-service teachers 'education, as well as implications for revised authorship theories and practical implications for supporting pre-service teachers as authors and composition instructors. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]