Security tools can help developers build more secure software systems by helping developers detect or fix security vulnerabilities in source code. However, developers do not always use these tools. In this paper, we investigate a number of social factors that impact developers' adoption decisions, based on a multidisciplinary field of research called diffusion of innovations. We conducted 42 one-on-one interviews with professional software developers, and our results suggest a number of ways in which security tool adoption depends on developers' social environments and on the channels through which information about tools is communicated. For example, some participants trusted developers with strong reputations on the Internet as much as they trust their colleagues for information about security tools.