Mobile botnets have recently evolved owing to the rapid growth of smartphone technologies. The implications of botnets have inspired attention from the academia and industry alike, which includes vendors, investors, hackers and researcher community. Above all, the capability of botnets is exploited in a wide range of criminal activities, such as, Distributed Denial of Service (DDoS) attacks, stealing business information, remote access, online/click fraud, phishing, malware distribution, spam emails, and building mobile devices for illegitimate exchange of information/materials. In this paper, we investigate mobile botnet attacks by exploring attack vectors and a subsequent presentation of a well-defined thematic taxonomy. Through identification of significant parameters from the taxonomy, we conduct a comparison to explore effects of existing mobile botnets on commercial as well as open source mobile operating system platforms. The parameters for comparison include mobile botnet architecture, platform, target audience, vulnerabilities/loopholes, operational impact and detection approaches. Related to our findings, we present open research challenges in this domain.