Given the numerous numbers of people who have Facebook, Twitter, or other social sites memberships, the chances that a "friend of a friend of a friend" may be able to hire said person, have the same interests as another does, or be able to help another in any way is pretty reasonable.
Networking is, essentially, the connections known to one, or not known to one, which can help them advance in their given sphere. As stated before, networking is not only about the people who one knows, it can be about those who one's friends know.
Since social sites have personal and business information on their members (which can be public or private), the chance to find a person with a given set of abilities is high. Also, most people tend to be friends with people who are similar to them (white collar with white collar/ blue collar with blue collar). If someone in the construction field is looking for a job, and many of their friends on a social site are construction workers, the chances of finding an opening through networking is high.
Therefore, one can use social sites to look around for other people, through their own friends and "friends of friends," who are associated with the business and careers they are interested in.