To study a network, we have to study both its topology (the graph) and its functionalities (tasks carried out on top of the graph). This chapter on topology-dependent influence models does indeed pursue both, as do the next two chapters.

A Short Answer

Started in October 2003 and formally founded in February 2004, Facebook has become the largest social network website, with 900 million users worldwide as of spring 2012 at the time of its IPO. Many links have been formed among these nodes, although it is not straightforward to define how many mutual activities on each other's wall constitute a “link.”

Founded in July 2006, Twitter attracted more than 500 million users in six years. At the end of 2011, over 250 million tweets were handled by Twitter each day. Twitter combines several functionalities into one platform: microblogging (with no more than 140 characters), group texting, and social networking (with one-way following relationships, i.e., directional links).

Facebook and Twitter are two of the most influential communication modes, especially among young people. For example, in summer 2011's east-coast earthquake in the USA, tweets traveled faster than the earthquake itself from Virginia to New York. They have also become a major mechanism in social organization. In summer 2009, Twitter was a significant force in how the Iranians organized themselves against the totalitarian regime.