In today's networked society where everybody and everything becomes inter-connected, it is very important to be able to identify key actors and key relationships in such a complex multi-layered eco-system. This paper focuses on the specific research challenge of identifying the most influential actors in a social network built through combining relationships among same actors in two different domains - communication domain (proxied through real-world mobile phone communication data) and social networking service domain (proxied through real-world Facebook data). A practical aspect of the paper is evaluated through the SmartSocial Platform, which uses methodology and implements algorithms that enable: i) joining multiple relations among actors across different social networks into the single unified social network; as well as ii) mining created unified social network for identification of most influential actors. Evaluation of the proposed approach is based on the social experiment with 465 users. Experiment results underline two important paper contributions: i) posting frequency sensitivity analysis shows a significant effect of posting frequency on social influence scores; and ii) interdependency analysis shows a synergic effect of combining data from communication and social networking service domains when it comes to calculating influence scores.