香港人口密度高是否形成一個社會問題?

Yes, it will be a social problem, if the population gets to be too large, where the government does not have enough money to accommodate to them, then there will be lots of negative consequences, such as medical, educational, housing, social welfare and jobs.

Medical: many people will rely on the public medical system, do we have enough medication, medical personnel, beds and other therapiest to cater to so many people?

Education: Are there enough schools for these kids, do they have to be in large classes, are there enough teachers to cater to them, does the government have enough recources to prvide free education to these kids?
Housing: Is the governmnet building enough accommodation so that these families have a shelter over their heads?

Social welfare: Are there enough recreation facilities for the young kids? As to prevent them from going into the wrong track.

Jobs: The job market, can it provide enough jobs for the population? Jobs from different hierarchy.

Sorry is not very comprehensive, but this all that I can think of for now.