Objective We investigate why certain candidates for the U.S. Senate are more likely to use social media for mobilization and fundraising. Methods Through content analysis of nearly 15,000 Facebook posts made by candidates for the U.S. Senate, we examine how candidate and campaign characteristics shape social media use. Results We find this type of campaigning is most common among challengers and Tea Party candidates who lack the name recognition and resources of more established candidates. Additionally, race characteristics, such as competitiveness and the relative positioning of candidates, influence social media posting strategies, with candidates in competitive races and candidates trailing their opponent more actively engaging in social media campaigning. Conclusions While nearly all candidates engage in some level of campaigning on social media, candidates in competitive races, challengers, and underdog candidates adopt these types of strategies most frequently on Facebook.