Social constructivism is a social theory of knowledge that applies the general philosophical constructionism of a culture into a social setting. When this knowledge is absorbed by the members of a culture, it is a shared common knowledge by the members of that culture, generally held to be true.
Social constructivism associates very well with scientific realism. Scientific realism is a direct attempt by the science community at-large to explain the things occurring in the physical world. The attainment of this body of knowledge, of how and why things work, are passed on in our homes, our schools, and generally regarded to be true.
There are always multiple ways of explaining things and this is where the anti-realists come in. They believe it would be a serious mistake to make the claim that all the answers we come up with as realists are correct. At one time, the Earth was believed to be flat; how true is that belief today?
Last, but not least, the metaphysical irrealistic philosophy tends to combine the "best of both worlds" by asserting each is a world version. Parts of the knowledge base of each are generally acceptable while parts are not so. The position might be summarized in colloquial terms as "Keep the part that works and let's fix the part that doesn't."