What is a Social System?

Ogbum and Nimkoff (channeling Talcot Parsons) have defined the term thusly:

A social system may be defined as a plurality of individuals interacting with each other according to shared cultural norms and meanings.

In our world-divided-into-nations, each nation has its own top-level social system. (A top-level social system has police power sufficient to enforce its laws internally, and military power sufficient to defend itself against other top-level social systems.)

The top-level social systems contain myriad sub-social-systems of wide variety (provinces, cities, corporations, transportation systems, central governments with their branches, protest movements, etc.).

Our planet does not yet have a top-level social system that is "all us, with no them".