Online social media (OSM) has a great influence in todays’ world. Some individuals view OSM as fertile ground for abuse and use it to disseminate misinformation, political propaganda, slander competitors, and spread spam. The crowdturfing industry employs large numbers of bots and human workers to manipulate OSM and misrepresent public opinion. The detection of online discussion topics manipulated by OSM abusers is an emerging problem attracting significant attention. In this paper we propose an approach for quantifying the authenticity of online discussions based on the similarity of OSM accounts participating in the discussion to known abusers and legitimate accounts. Our method uses multiple similarity functions for the analysis and classification of OSM accounts. The proposed methods are demonstrated using Twitter data collected for this study and previously published Arabic Honeypots data. The former includes manually labeled accounts and abusers who participated in crowdturfing platforms. Demonstration of the topic’s authenticity, derived from account similarity functions, shows that the suggested approach is effective for discriminating between topics that were strongly promoted by abusers and topics that attracted authentic public interest.