Based on your title, I thought your question would be fundamentally unanswerable, but upon opening it, I think there are some pretty fair answers that can be given within two limits:
- As post-modernism is being used here as fuzzy term, I can't and don't claim that the answer reflects the view of postmodernism in general or even the thinkers I am using to answer.
- I am going to answer the question as a variant of "how do post-modern theories in philosophy understand social change?"
Firstly, isn't the very definition of a society or culture the collective ideals and views of a group of people? Therefore, how is a society distinguishable at all from the people that constitute it?
That is an interesting question really. First off, you are mixing "society" and "culture"; let's read this charitably and just use "society." Second, you are saying society is the same as people's ideas. But in the next sentence you are saying it's people.
A major point of many "post-modern" thinkers which they inherit from Heidegger (inter alia) is that this conflation is false. Put another way, the ideas floating around differ from who we are. For Heidegger, this is part of the ontic ontological distinction, a distinction between the ontological categories we apply to things and their ontic realities.
Foucault inherits this and does a good part of his work looking at the concepts we use to frame society and how they change over time. Discipline and Punish for instance is about the way we switched from physical to psychological punishments and incarceration as our ideas about the purpose of punishment and its goals shifted.
With that in hand, we can move forward:
For if a society is shaped by the viewpoints of the people in it, and the people in a society are shaped by the society itself, then clearly they must either together be one single entity, or we must conclude that the above definitions of the individual and society do not make any sense at all.
I actually don't follow this argument whether we are using classical philosophy, medieval philosophy, modern philosophy, or post-modern philosophy. Starting with Aristotle, we can see individuals as a part of a larger social whole the state) -- that does not delete either or make either superfluous. Nor once we have more complicated ideas of how ideas change over time are we required to conflate:
- The ideas themselves
- The people who have some (or all) of a set of ideas.
- The changes in ideas over time that a person possesses.
- The ideas the society possesses at any given time....
Responding more generally, the post-moderns are in general post-structuralists rather than pure structuralists. The distinction usually doesn't matter but here it does. Even if post-moderns believe individuals are cogs in the motion of social ideas rather than independent agents (a deeply disputable claim), they are the sort who believe these ideas are changing and turning. This is in fact central to many of them:
- Ronald Barthes - death of the author is the idea that it doesn't matter who came up with ideas; instead, they have a life of their own
- Derrida's use of the notion of bricolage or his consideration of justice hinge precisely on the belief that ideas can change over time.
- The same can be said of Foucault's work
If we want to count Sartre among the post-modernists, then a major point of his view is that values change -- and they change because we are radically free -- rather than accepting any of the concepts of any society as fixed.
Simone de Beauvior similarly works from the idea that pernicious ideas in society are changeable and had bad origins.
With that further consideration, let's look at:
And since that society is made up of and defined by many such entities, which all are incapable of concieving of an idea outside that society's parameters, and thus are incapable of changing that society, the society can never changes; it is forever static.
This strikes me as a fair interpretation of a structuralism that perhaps no one ever held to, but the post-moderns definitely do not hold to it. Particularly, I think you are right about one premise: for post-modernism, people are not freely choosing their values.
From this, you draw a conclusion that is not accurate. This does not mean that they have static values. Instead, as Merold Westphal describes them, "they are holists without the whole." So it's like Hegel's idea of spirit -- there's social change happening but without any direction and without any final destination.
Worded slightly differently, every individual in a society is not on any post-modern view required to have all of the same values that come from a society. Instead, they can all have slightly different (mostly non-free) sets of values and these can interact and evolve over time and be taken over by new bearers who are part of the society in different configurations.
There's a lot of critiques one can raise against the postmodernists -- such as the loss of ethics, but the idea that post-modernism is incompatible with changes in the values of a society over time isn't really a strong one.