When companies take stances on controversial sociopolitical issues, diverse publics form divided responses and engage positively or negatively, actively or passively with the focal companies in the online discourse. This dissertation took a cross-disciplinary approach to investigate this corporate communication practice, corporate social advocacy. First, Study 1 employed a quantitative content analysis study to explore the existing communication strategies (relational vs. elaborational vs. activational) used in companies’ CSA social media. The results showed that the most commonly used communication strategy (elaborational) did not appear to be the most effective one in facilitating publics’ social media engagement behaviors. Instead, using a relational communication strategy in a CSA message to build explicit linkages to corporate images or functions was often associated with a higher level of publics’ social media engagement. Meanwhile, existing activational communication strategies in CSA communication tended to be general and ineffective in enhancing publics’ engagement and participation. Second, In Study 2, a pilot study and a main study were conducted to examine the impacts of advocacy fit and social identities on publics’ CSA attributions, attitudes toward the company, and social media engagement intentions. A holistic measurement of social media engagement was empirically validated by covering the activeness and valences of engagement in the pilot study. The main study showed that congruency between a company and its CSA, especially image-based, contributed to more perceived value-driven and less egoistic and strategic motives, which, in turn, led to more positive attitudes toward the company and desired social media engagement intentions. Moreover, Study 2 introduced publics’ social identities to explicate their responses to CSA. Participants’ social group membership and ingroup identification were significant factors in explaining their CSA attributions, attitudinal responses, and social media engagement behaviors. Additionally, Study 2 demonstrated that social group membership and ingroup identification could function as antecedents for publics’ situational perceptions of sociopolitical issues, offering additional ways to identify and categorize publics. This dissertation is theoretically and practically valuable in terms of several aspects. First, it reinforced the imperative role of communication in CSA with empirical evidence about the communication strategies across various companies on social media. Second, investigating the effects of advocacy fit on attributions guides strategic CSA communication that needs to align organizational identities and sociopolitical issues. Third, by incorporating the social identity approach (Tajfel & Turner, 1979; Turner, 1985), this dissertation moves the theorizing publics forward with the additional considerations of societal-level factors, such as power structure and intergroup dynamics. Fourth, the comprehensive measurement of social media engagement intentions contributes to the public relations literature, given the central role of social media engagement in building and maintaining organization-public relationships (Lim & Young, 2021).