Although educators are excited about the potential of social network sites for language learning (SNSLL), there is a lack of understanding of how SNSLL can be used to facilitate teaching and learning for English as Second language (ESL) instructors and students. The purpose of this study was to examine the affordances of four selected SNSLL (LingQ, Lang-8, italki, and Polyglotclub) through site design and user experience from two perspectives: using the sites as a language teacher through site analyses; and using the sites as a language learner through usability testing. The findings showed that these sites have the potential to facilitate language learning. The learning tasks created using the social networking features of these sites are to provide examples of how to use these SNSLL as teaching tools under a socio-constructivist learning framework. In addition, the results of the usability testing highlighted the importance of considering the website's information, interface, and interaction designs. SNSLL should be as easy to use and as intuitive to navigate as possible. We hope the findings will provide valuable insights regarding how SNSLL can be used as teaching and learning tools, and how they should be designed to facilitate pedagogical goals.