Social comparison is a common focus in discussions of online social media use, and differences in its frequency, causes, and outcomes may arise from country or cultural differences. To understand how these differences play a role in experiences of social comparison on Facebook, a survey of 37,729 people across 18 countries was paired with respondents' activity on Facebook. The findings were augmented with 39 in-person interviews in three countries. Social comparison frequency was more strongly predicted by country than by age, gender, and Facebook activity combined, indicating that country differences are important to consider when studying social comparison. Women's and men's experiences differed greatly between countries. Exposure to high feedback counts on friends' posts was associated with more frequent social comparison, but only in some countries. Design interventions that account for such country differences may be more effective at reducing the negative outcomes of social comparison.