Definition of social network

This is a social structure consisting of individuals or groups who are connected to each other, for example through friendships.  When these connections are represented in a database and with a web interface, it is usually referred to as a “social network service” or simply “social networking”.

Social network services drastically reduce the barriers of entry and became extremely popular in the second half of the first decade of this century, as more people started to consider the web as the perfect place to keep in touch with their family, friends or acquaintances.  For example, Facebook, a social networking site that allows you to keep in touch with people in your life, reached 500 million active worldwide users in July 2010.

Despite criticism,  generally social networks are not substituting “normal” life, but complementing it.

Example
Social network services can be horizontal - for general use and with no clear purpose stated - or vertical - devoted to a specific activity or interest, as in “professional social networks” such as LinkedIn or dating services. [1]