“…Popular social sites such as Facebook, Pinterest and Twitter have undergone explosive growth and are turning into community spaces, where users interact with their friends and acquaintances. With the numbers of active users on these sites numbering in the millions or even tens of millions, how to identify and measure the influential users and topics becomes an important problem with applications in marketing (Kempe, Kleinberg, & Tardos, 2003), information dissemination (Gruhl & Liben-nowell, 2004;Leskovec et al, 2007), social relation visualization (Kim, Ji, & Park, 2014), and expertise discovery (Davitz, Yu, Basu, Gutelius, & Harris, 2007).…”