Citigroup Inc. (C.N: Quote, Profile, Research) on Monday said computer tapes containing account data on 3.9 million customers, including Social Security numbers, were lost by United Parcel Service Inc. (UPS.N: Quote, Profile, Research) .MCI Inc. (MCIP.O: Quote, Profile, Research) last month lost a laptop that stores Social Security numbers of 16,500 current and former employees.