As information is massively consumed and exchanged over the Internet and social media in particular, governments and nongovernmental entities see the potential to influence public opinion via these platforms. From a conceptual standpoint, communication scholars have studied the agenda that is transferred from traditional media to the public since the 1970s. Recognizing that social media allows multiple and competing agendas to emerge, this essay proposes a conceptual and methodological approach to identifying agendas, their sources and evaluate their success in terms of transfer of agenda salience to the public. Taking a semantic networks approach, we conceptualize agenda as a set of issues and attributes that are mentioned together in social media posts. The extent to which such issues and attributes are found among the public discussion, then, allows us to evaluate the success of public opinion influence. Specifically, it allows us to identify the subgroups of social media users that are more susceptible to influence, via information, disinformation and mal-information.