  After the Internet and the World Wide Web have become popular and
widely-available, the electronically stored online interactions of individuals
have fast emerged as a challenge for researchers and, perhaps even faster, as a
source of valuable information for entrepreneurs. We now have detailed records
of informal friendship relations in social networks, purchases on e-commerce
sites, various sorts of information being sent from one user to another, online
collections of web bookmarks, and many other data sets that allow us to pose
questions that are of interest from both academical and commercial point of
view. For example, which other users of a social network you might want to be
friend with? Which other items you might be interested to purchase? Who are the
most influential users in a network? Which web page you might want to visit
next? All these questions are not only interesting per se but the answers to
them may help entrepreneurs provide better service to their customers and,
ultimately, increase their profits.
