I think you're missing the point. The question isn't whether concepts refer to existing things or not, but whether they are valid and useful.
Nothing general ever exists - only the particular. The number two doesn't exist, although you can find twos in nature. Basic Aristotle. Similarly, race doesn't exist, but you can find instances of races in nature.
That's why arguing that a concept is invalid because it is "only" a social construct is disingenuous. Concepts are always constructs; they are never "natural" in themselves. What determines their validity is their usefulness, which is related to how closely they mirror the structure of human experience.