Vietnamese high school students have few opportunities to use English outside class and they are often reluctant to speak in class. This paper describes and explains the students' willingness to communicate (WTC) and relates this to varied perceptions of social presence. Eighteen high school students in Vietnam took a six-week online course using Facebook and Skype. They were interviewed individually before and after the course about their experiences, focusing on their perceptions of their own WTC. The results show that the students were more willing to use English spontaneously in the online environment in contexts where they perceived that they had less social presence. Text and audio chat were felt to be less face threatening than video chat, and consequently, students were more willing to speak in conditions of lower social presence. It can be concluded that the more social presence students felt they had in the online environment, the less their WTC. This was true for both synchronous and asynchronous online environments. Allowing students to control their social presence in online communication can embolden shy students and increase their WTC.