I'm not familiar enough with the details of the black-footed ferrets to speak on that one. But I do know there are numerous species of animals that are completely extinct in the wild, and still exist only due to captive breeding. In lots of cases, it is captive breeding done in accredited zoos, but in others it is due to private keepers. There are at least a few examples where captive breeding programs have allowed animals to be successfully reestablished in the wild.