Recently, I told my oft-absent roommate and friend of many years that I'm tired of being seceretary to all the people who call here looking for him while he's quasi-secretly living with girlfriend. To this, my roommate's girlfriend replied (for him) that he has a life and the callers need to understand that. Which made me think: he doesn't have a life, in fact he has about the most pathetic social life of anyone I know; what he has is a girlfriend that takes up all his time. I argue that there is a big difference.
A social life involves being surrounded by a variety of people, not necessarily new people always, but different people at different times. One could meet new people all the time, or have many groups of people who he or she hangs out with frequently. I don't think anyone's definition of social life involves having one person who you are with all of the time with no room for anyone else.
Such a person holding a monopoly on someone else's time, aside from creating unhealthy codependance, also kills any previously existing social life because to give so much time to that one person for such a prolonged period is to effectively ditch all other friends. This is exhibited by all of the people who call here asking where my roommate is, why he hasn't called, and even go as far as asking what's happening in his life, because he's never available to tell them himself. In addition, the little time my roommate spends in truly social activities is unpleasant time away from his girlfriend.
Most people use the ratio of time spent out of the house to time spent at home as the test for a whether someone "has a life" or not. It is this popular test that makes someone believe that being outside of the house with one person all the time is the same as having a social life. But this test is fatally flawed because it assumes that time spent outside the house is always spent in social activity, which it is not.
Well, you say, he may be in love. That is fine with me, because it's not what I am arguing. I'm just saying that spending all your time with a significant other is not a social life, it is occupying your time with another person without necessarily being social. You can be with a person and be social with them, but being with that person in itself is not social. Just because you have a girlfriend doesn't mean you have a social life.