A social life is the term used in referring to a person's dealings with other people as a generalization.
Those with "no social life" are those grouped into the category of people with no or very few friends, and who are rarely seen with other people, especially at popular social functions such as parties or sports events. People referred to as not having a social life may, in fact, have plenty of dealings with other people, but they are probably just more private or fringe, and thus not talked about or accepted (in many cases because they do not involve people outside said person's regular group of affiliates).
The only prerequisite for having a social life, in American society at least, is a number of well-known friends and usually a good relationship (or a mimic thereof) with those outside one's close circle of friends. Noticable attendance at social events that attract large numbers of people, particularly from outside of one's usual clique, also helps in establishing the existence of a social life.
Unfortunately, those who have "no social life" often have difficulty in gaining one, because one usually must have a social life in the eyes of his peers in order to be deemed worthy of attending functions which would establish the creation of said life. In other words, sometimes the prerequisite of gaining a social life is already having one, which proves a subtle journey for those who have not been gifted with it early on.
Through these elaborations, "a social life" might be defined as a black and white term for people who either have or do not have good social skill with different groups of people, rather than a literal "second life." So I would say that the creation of the social life requires attendance at such social functions as parties on a regular basis in order to demonstrate a skill and willingness to interact with other people, but is not representative of those exact events.