The state of a city and it's culture when it's population is too great and the subsequent infrastructural/behavioral problems that are associated with excess become obvious. Social decay occurs and can be seen on both the physical level of everyday life in a city (abandoned buildings, vacant collapsing houses, streets in poor condition, etc) and on the emotional state of it's inhabitants (narcissism, social anxiety, paranoia, etc). Typically people are generalized into a vast and generic group called "strangers" and these strangers are often ignored completely (by other strangers) in order to keep the city running efficiently and problems associated with poor behavior suppressed. The only time strangers interact with each other is when one stranger offers another stranger a service or something that the other stranger needs or wants.
Examples of Social Decay:
Stranger A: Would you like a bag for your food?
Stranger B: Yes
Stranger A: Would you like to help out the homeless today?
Stranger B: No
Stranger A: Hey baby, can I buy you a drink?
Stranger B: Sure
Stranger A: Can I sit here?
Stranger B: I don't know man, it's too hard to tell!
People that are afraid to leave their apartment because of fear of strangers.
People that feel they are eroding but only as an expression of what they see around them.
The overall and overwhelming sensation that everything is collapsing and must be destroyed to start anew.
A car on fire.
Police shooting protestors.