I. Introduction
In recent years, online social networking service has become popular. An online social networking service is a popular web-based service which provides a platform for people to create profiles and share these profiles with other people. With this service, people can share their daily life and their opinions with their friends on the Internet. Some online social networking sites such as MySpace, Facebook, Google+, and Bebo are popular and have attracted millions of users [1], [2]. Some people even register on several online social networking sites. In this work, we focus on a set of users who have accounts of three online social networking sites, namely Facebook, Foursquare and Twitter. For each website, users and the connections between them can constitute a network. We call the networks of these online social networking sites the “online social networks”.