  All major on-line social networks, such as MySpace, Facebook, LiveJournal,
and Orkut, are built around the concept of friendship. It is not uncommon for a
social network participant to have over 100 friends. A natural question arises:
are they all real friends of hers, or does she mean something different when
she calls them "friends?" Speaking in other words, what is the relationship
between off-line (real, traditional) friendship and its on-line (virtual)
namesake? In this paper, we use sociological data to suggest that there is a
significant difference between the concepts of virtual and real friendships. We
further investigate the structure of on-line friendship and observe that it
follows the Pareto (or double Pareto) distribution and is subject to age
stratification but not to gender segregation. We introduce the concept of
digital personality that quantifies the willingness of a social network
participant to engage in virtual friendships.
