Social circles are groups of socially interconnected people. A Social circle is distinguished from a social pyramid in that there are two perspectives that can be used to describe a social circle: the perspective of an individual who is the locus of a particular group of socially interconnected people; and the aggregate perspective of a group of socially interconnected people. While asocial pyramid considers all of the people with whom an individual has contact and defines a structure to support and empower (or disempower) that person; a Social Circle can have narrowly defined boundaries, often all of the members of the circle have been in contact with each other, and there may be a process of social initiation required for people to gain membership. It has been said that what you bring to your Social Circle is what you create in your social pyramid.
Social circles are interconnected through social connections between individuals. The theory of six degree of separations points out that mathematically, there are on average 6 steps between any two living people on Earth.
Social circles also overlap. For example, there may be people who work together who are also in the same tennis club or coffee clatch, or you may know someone independently of a mutual friend.
Social circles tend to have unique sets of norms and values. When an individual does not comply with them, the individual may be ostracized, admonished, or even embarrassed by other members of the group. Hierarchical social structures with socially-defined positions provide an excellent example how norms and values are applied within social circles. For example, a social circle could be a Toastmaster club, where there are executive positions that are held by a clearly defined process. Alternatively, a social circle could be a group of Cricket enthusiasts who gather in a local pub once a week to watch a game. In the second case, group leadership and membership is informal in nature.
source : http://en.wikipedia.org/wiki/Social_circle