Social media with diverse functionalities enables people to communicate, make connections, exchange information, and transfer knowledge without time and location restrictions. The number of informal learning communities based on social media is growing, but the knowledge about how these communities work is still limited. Three main issues for research emerging from limited literature are related to participation, community, and learning. This study used the Community of Practice framework to investigate differences between active community members and lurkers in terms of their participation and perceptions of community and learning. 82 participants (43 active members and 39 lurkers) were recruited from Reddit-based education-related communities. An explanatory sequential mixed-methods design was used to collect both quantitative (a survey about the sense of community and perceived learning) and qualitative data (interviews with 9 participants: 5 active members and 4 lurkers). Although the quantitative data suggested that there was no significant difference between the perceptions of lurkers and active members, qualitative findings suggested they had different perceptions of communities, especially in terms of their negative impressions. Lurkers tended to be more negative about the technical features supporting communities (i.e., Karma points and up/down votes), while active members tended to be more negative about people who hindered community building through disrespectful or unkind contributions. In addition, both lurkers and active members reported that informal social-based communities contributed to their learning, but the learning described by active members tended to align with higher levels on Bloom's taxonomy. Practical implications are provided for online community managers and educators in terms of reconsidering design and management in informal social media-based communities to support both lurkers and active members. A discussion of similarities and distinctions between this study's findings and findings from formal online learning communities is also provided as recommendations for future research. [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.]