If the amount of cargo is not evenly distributed across a network a freighter will sometimes have to fly a route not loaded to capacity. Do cargo freighters ever have to be ferried empty to alleviate this "back-haul problem"? The link uses the example of trucking, but my question is to what extent this issue manifests itself in scheduled air cargo (e.g. FedEx, UPS, etc.).

(Note: My assumption is that this affects scheduled air cargo operators more severely than specialty cargo operators doing "one-off" hauling of special goods)