Wikis, as one of the Web 2.0 social networking tools, have been increasingly integrated into second language (L2) instruction to promote collaborative writing. This paper reports on a case study involving 35 university students at the beginning level who contributed to wiki pages over a period of 14 weeks. The affordances and constraints of using wikis for collaborative writing were drawn from data triangulation: (a) group wiki pages, (b) student surveys, and (c) final interviews. The results show that creating wikis had a positive impact on the development of students' writing skills through collaborative engagement. Scaffolding through peer feedback played a crucial role in the L2 writing process through which students not only helped each other organize the content but also made error corrections for language accuracy. In addition, the results indicate that task type affected the amount of writing produced by each group. The study suggests that the instructor needs to guide students during the revision process and offer them hints and tips for effective use of feedback. (Contains 2 tables, 2 figures and 9 notes.)