  The aim of this article is to provide an understanding of social networks as
a useful addition to the standard tool-box of techniques used by system
designers. To this end, we give examples of how data about social links have
been collected and used in di erent application contexts. We develop a broad
taxonomy-based overview of common properties of social networks, review how
they might be used in di erent applications, and point out potential pitfalls
where appropriate. We propose a framework, distinguishing between two main
types of social network-based user selection-personalised user selection which
identi es target users who may be relevant for a given source node, using the
social network around the source as a context, and generic user selection or
group delimitation, which lters for a set of users who satisfy a set of
application requirements based on their social properties. Using this
framework, we survey applications of social networks in three typical kinds of
application scenarios: recommender systems, content-sharing systems (e.g., P2P
or video streaming), and systems which defend against users who abuse the
system (e.g., spam or sybil attacks). In each case, we discuss potential
directions for future research that involve using social network properties.
