When is a city not a city?

This one is perhaps related to " When cities become too big?", but with a twist:

It's been a heckuva ride for cities (and the civilizations they have created) from the rise of Mesopotamia to our giant global cities today.

The concept of city grows, changes, redefines itself...yet our cities still remain cities.

But I have to wonder: are we entering an era when some of our cities (alone and with their their surrounding metropolitan areas) grow so large in population (with growth in area to make it necessary) that suddenly they become something else, something more impersonal....but not a city?

Considering our concept of cities and how they function, how their citizens relate to the same common place and have an interrelationship and a sense of place that you won't find in more artifical political units as state and nation, does a city reach such a size that it no longer functions as a city?

Can 30,000,000 or 40,000,000 people really live in one municipality and have any unity that makes a city a city? any knowledge of each other's lives. and common sense of place?>