With regard to Facebook (what the article focused on), the right generalization seems to be that virtual social networks do, in fact, largely reflect actual social networks https://www.facebook.com/notes/facebook-data-team/anatomy-of...

The article struck me as paranoid, but I agree with your assessment that the authors' reserving their kids' digital real estate is not contradictory with the authors' concern -- it leaves the choice up to the child.