Culture shapes interpersonal communication. However, little is known about how culture interacts with computer-mediated communication, CMC) tools in a business context. We present a large-scale empirical study of cultural differences in computer mediated social interactions in a global company. Our dataset includes 9,000 volunteer users and more than 20 million records of their email and Instant Messaging conversations. We compared social network characteristics, preferences for CMC tools, and expression of sentiment across employees working in seven countries. Significant differences emerged and the patterns are consistent with the inherent cultural characteristics as suggested by cultural theories. In addition, we uncover the complex manner in which culture interacts with preference and use of different communication mediums. The existence of pervasive and complex cultural differences, points to the need to understand and account for such differences in designing cross-cultural collaborative systems.