Truths that do not arrive at Santa Claus
Truth as Social Construction
"Yes, well, what about the sun, the moon, the stars."
You're right of course. No human being constructed these things. Still, the naming of the stars, the differentiation between the sun and other stars, calling the moon at one time "full" and at other times "new" these are all social constructions. There is almost nothing that we can experience that is not in some way socially constructed. When the moon is full, it is full because that's what we agree to call it. If you don't call it that, you are wrong. The reason you are wrong is because we all say so, simple as that.
Still, if this were the only way in which Santa were real, it would be kind of sad. There is at least one way that Santa is real than just this.