Social constructions are socially and subjectively created facts: a social construction labels some entity that exists only in the minds of (most) people in society. The most obvious example of a social construction is an agreement between two people. The agreement itself does not name any entity in the external, objective world (the physical world that does not include our minds); it names just the fact that some particular mental state is shared between the two people.
There's nothing weird or objectionable about social constructs; we've been creating social constructs since we began speaking. Indeed the primary function of speech is to create social constructs; although of undeniable importance, the utility of speech for communicating information about the external world is only secondary.
Ownership, as distinct from possession, is a social construct. Ownership might reference possession, but ownership and possession are different concepts. Possession is a physical fact; ownership is a shared idea, an idea in the minds of individual members of a society. Even were we to all agree that possession, and only possession, entails ownership, possession would label the physical fact of having some object in a person's physical control, an external fact; ownership would label the ideas in our minds that physical possession was legitimate.
Libertarianism (with absentee ownership) defines a specific social construction of ownership, one that does not correspond to possession. As best I can figure out, Libertarianism establishes ownership as only the unbroken chain of voluntary transfer of property from the original creator to the current owner, and the near-absolute power of the owners of property to use, not use, or destroy that property as they please.
In contrast, non-Libertarian philosophies construct ownership as in some way departing from either or both of the transfer principle or the absolute use principle. Liberal capitalism, for example, permits taxation (i.e. the government may legitimately compel the transfer of some property). Communism, as another example, forbids the private ownership of the means of production; the means of production may no more be privately owned than a human being.
It's more difficult to decide between social constructions than external referents. Both are, in a sense, socially created (e.g. we have simply agreed that the word "mass" refers to an object's tendency to resist acceleration), but a social construction does not refer to anything in the external world, and we cannot distinguish between conflicting social constructions directly on a scientific basis.
Hence the oft-repeated question: why should we prefer to legitimatize the Libertarian social construction of ownership to alternative social constructions of ownership? It cannot be that the Libertarian construction is scientifically determinable, because "ownership comes from the unbroken voluntary transfer from the original creator" does not directly reference anything in the external world, the world we can — if we are to believe the scientists — made decisions about using the evidence of our senses.
We can, of course, always appeal to pragmatism, an indirect scientific justification. We can say that legitimatizing this or that social construction will have specific external, scientifically determinable results, results we find clearly more or less preferable than other results.
The big problem with Libertarianism is that the deontic and the consequentialist justifications for Libertarianism both seem weak. Since Libertarianism is a social construct, not directly scientifically justifiable, I can't see any way to justify Libertarianism deontically by anything but argument by vehement assertion. And the pragmatic justification seems to rely on at best naive optimism and at worst a willful ignorance of facts. (The most administratively efficient* health care systems in the United States, for example, are Medicare/Medicaid and the Veterans Administration.) When the weaknesses in the pragmatic justification become apparent, Libertarians almost always seem to switch instantly to the deontic justification: whether or not it works "better" than current systems, the Libertarian version is intrinsically better because people will have more "choice." Of course, it would be great if any ideology were both deontically and consequentially preferable, but they seem to try to use each justification to shore up weaknesses in the other justification.
The switch between the deontic and pragmatic justification to shore up (and at worst distract) weaknesses in the other justification is most reminiscent of Christian Apologists. (While religious apologists hold no monopoly on fundamental defects of justification, they are the most vigorous and numerous examples.)
To a skeptic, if an argument is good enough to use to support a proposition, then its failure contradicts the proposition, or at least calls the truth of that proposition into serious question. If I believe something because of an argument, then if the argument fails the belief should fail too. If I believe something not because of some particular argument, then it seems dishonest to use that argument to support the belief.
For example, I believe that communism is better than capitalism because I believe the social ownership of the means of production would lead to more happiness and satisfaction in the world. I justify my communism on purely pragmatic grounds. If someone shows me that communism, or some aspect thereof, would lead to a worse outcome, then I will change my conception. For example, we know from experience that while it might be effective at quickly industrializing a poorly-industrialized country, unconstrained political rule by a self-selecting party elite has serious negative consequences when industrialization reaches a certain stage. I therefore abandon the idea of such a party elite. If the theory does not fit the facts, the theory must change.
An apologist, on the other hand, in some sense "just believes." The arguments and reasons proffered really have nothing to do with the foundation of the apologist's beliefs; the arguments are tools only of persuasion, not justification. Either the belief is mystical, revelatory, or the actual foundations of belief are at best subconscious and at worst intentionally covert.
Hence, to a certain extent, arguing with apologists is an exercise in futility. A rebuttal has no effect; they do no believe because of the argument, so even a decisive rebuttal does not undermine the belief. Furthermore, even showing that the argumentation is disingenuous in this sense is such a subtle point that only someone with considerable education, training, and experience in critical thinking and rational belief formation can spot the disingenuity, and those people are already strongly predisposed against mystical or covert belief.
Really, the only value in taking to apologists, religious, political, or ideological, is to try to gain insight into good questions to ask; even though their answers are defective, the questions are almost always valuable. Indeed, a good question is a thousand times better than a good answer. (Happily, there are a thousand times as many good answers as their are good questions.)