The general definition of social sustainability is the ability of a social system, such as a country, to function at a defined level of social well-being indefinitely. That level should be defined in relation to the goal of Homo sapiens, which is (or should be) to optimise the quality of life for those living and their descendants.
After that, there is universal disagreement on what quality of life goals should be. Not only do nations disagree. So do their political parties, their religions, their cultures, their classes, their activists organisations, and so on.
Therefore there is no sense to define what quality of life goals should be, even in the broadest sense. This means that social sustainability on a practical, implementable basis is undefined. Thus it’s the weakest pillar of them all because people can’t even agree on which way is up. This is a shame because a strong social pillar is the topmost goal of democratic systems.