For Context, I am reading http://web.mit.edu/rpindyck/www/Courses/NetEx19.pdf. The chapter talks about network externalities in single system like fax machine, email, vaccinations for measles. Paraphrasing from the book "In each of these cases, the marginal social benefit of one more person joining the “network” is greater than the marginal private benefit. Hence, the equilibrium network size can be less than the socially optimal size, so that the competitive market will not be efficient".
Why is equilibrium network size less than socially optimal?