Social sustainability goes a level further in abstraction than economic sustainability. For example, if a system can provide a livable basic income for everyone in a sustainable manner, than that system might be economically sustainable. However, if a basic income is insufficient to keep most of the population satisfied, then there will be a source of social instability. It that instability cannot be contained or counter-balanced, then the society itself is unsustainable.
If people develop unrealistic expectations for a society or a distorted view of scientific reality, then they may place disruptive pressures on a society.