The effects of social annotation on critical literacy remain controversial, and little research has explained why some students can benefit from social annotation in critical literacy, and some cannot. This chapter investigates the effects of social annotation on critical literacy and the interactive patterns of social annotations. The participants were 45 English as Foreign Language college students of low to intermediate reading proficiency level in Taiwan. The collected data included (a) students' responses to reading comprehension tests and (b) students' social annotations on selected reading texts. We evaluated critical literacy in terms of three skills: identifying main ideas of text information, evaluating the accuracy of text information, and synthesizing text information through Analysis of Variance. The results showed that social annotation facilitates the development of critical literacy in identifying main ideas and evaluating text information, but not in synthesizing text information, because the information retrieved from social annotations is often scattered. The interactive patterns of the social annotations were categorized into the discussions of (a) the meaning of vocabulary or phrases, (b) the use of vocabulary, (c) the detailed text information, (d) the grammatical structures, and (e) the main idea of paragraphs. Pedagogical implications are drawn from the five interactive patterns of social annotations. [For the complete volume, "Educational Media and Technology Yearbook, Volume 42," see ED600543.]