The purpose of this study was to determine if the MathTwitterBlogosphere (MTBoS), an online community of practice, provides opportunities for mathematics educators to learn about the knowledge and conceptions that support the effective use of cognitively demanding tasks, including mathematical knowledge for teaching, visions of high-quality mathematics instruction, and views about how to support struggling students. A qualitative case study was used to explore how content within the MTBoS, and perceptions of the effects of participation in the MTBoS, related to learning specified knowledge and conceptions. Results suggest the MTBoS is a promising avenue for providing learning opportunities related to mathematical knowledge for teaching and visions of high-quality mathematics instruction, although it may provide fewer learning opportunities related to engaging struggling students.