We empirically study the motivations of users to contribute content to social media in the context of the popular microblogging site Twitter. We focus on non-commercial users who do not benefit financially from their contributions. Previous literature suggests two main possible sources of motivation to post content for these users: intrinsic motivation and image-related motivation. We leverage the fact that these two types of motivation give rise to different predictions as to whether users should increase their contributions when their number of followers (audience size) increases. In order to address the issue that the number of followers is endogenous, we conducted a field experiment in which we exogenously increased this number for a set of users (treatment group) over a period of time and observed the impact on their posting activities compared to a control group. We find evidence for both types of motivation. We estimate each treated user's utility function using a dynamic discrete choice model.