Prologis, the largest U.S. owner of industrial real estate, said in a May report that utilization rates, which indicate how much space is being used, reached close to 85%. Vacancy rates are at 4.7%, close to a record low, the company said. Amazon is the real estate firm's biggest customer, occupying 22 million square feet, followed by Home Depot at 9 million and then FedEx and UPS, according to Prologis' latest annual report. Walmart is seventh. In April, an analyst on Prologis' earnings call asked what types of clients were most actively pursuing leases.