Abstract
Online Social Networks (OSNs) have become fundamental parts of our online lives, and their popularity is increasing at a surprising rate every day. However, besides the revolution the OSNs have generated in social networking, they have also introduced some issues; first, since the amount of multimedia data on the Internet is growing continuously, it is extremely important for users not only to share multimedia content with each other, but also to receive the specific content they are interested in; second, OSNs have introduced new threats to their users due to their attractiveness, the ever-increasing number of users, and the massive amount of personal information they share. For such reasons, in this paper we propose a survey of online social networks, which can hopefully support both researchers and social network users. More specifically, we focus our attention on the most relevant research challenges regarding semantics and security.