My understanding is that social Darwinism refers not to human programs such as eugenics and genocides, but to "naturalisitic" processes (unplanned and unintended) such as the survival of the best of two restaurants, for example, competing for scarce resources (customers in a market that can support only one of them - perhaps a small town).
The one that is more efficient, that is, offers the best combination of food, service, ambiance, prices, and convenience, will survive - prosper even - as the other goes extinct. That is the cultural equivalent of biological fitness.
That, Adam Smith's invisible hand, is much closer to Darwin's theory than what Darwin's detractors call social Darwinism. And there, too, it is a powerful process that leads to net improvement over time without plan or effort.