In our culture (I guess as in most) we have introverts and extroverts. Introverts have to think things out in their heads, need time to think quietly about things before they act or speak about it, when their social energy is used up they need to be alone to rejuvenate, may have only one or two friends, and dislike social gathering expect with close friends. Introverts tend to isolate naturally. Extroverts are the reverse; they process decisions by talking, have to be with people to rejuvenate, have many friends, and thrive in social settings. This is not an either or situation. We see this on a continuum; very introverted people at one end and very extroverted at the opposite end. In the middle it might be difficult to tell the difference.
Depending on where your introverted teenager falls on the continuum introversion may be a variable in social isolation. Many times people that are introverted wish it was different. This is most true in adolescence. However, at this time, introverted individuals are more outgoing than most other times in their lives. In the October 17, 2011 blog "Teenagers only think with half their brain….." we discussed physiological changes in the adolescent brain and how this increases the need to socialize with peers. As a result even the introverted are driven for peer involvement. It will be with smaller groups or just with another individual. It will be less frequent that the extroverted teenager but they will still need to be with peers to some degree. On line peer group contact is not the same as face to face even though many will argue that "that is my peer group".
If the social isolation if fairly constant there are likely other issues complicating the situation. Then you could have a component of social anxiety low self-esteem etc. If they complain to you that they have no friends you might help the introverted teen find small, like interest groups that are available in your community. They could join a theater group, an Explorer group or various other activities. An introverted teenager does not necessarily need professional help only if other variables are present. If you suspect you teenager is introverted and you are not do a bit of research to learn more about this. Sherri