While the popularity of Social Network Services (SNS) has been growing, little research has been conducted on the application of SNS in EFL learning. The purpose of this study is to investigate the effectiveness of English writing activities using SNS, especially Twitter, on students` writing abilities and its effects on their affective domains. A total of 74 university students were assigned to two groups and given nine different writing tasks to complete either on paper or through Twitter over a period of nine weeks. Each student`s writing sample was assessed at the first week and the last week of the experiment as a pre-test and a post-test. The participants` impressions of using Twitter in their writing tasks were also investigated through questionnaires. The results of the study show that the group that used Twitter for their writing activities showed significantly greater improvements than the group that used paper. The results of the questionnaires show that students in general hold negative attitudes towards engaging in writing activities using Twitter. The reasons for this may be the lack of familiarity with the learning activity presentations. Based on the results, this study suggests that the development of appropriate writing activities using SNS is needed.