My last check from my employer this year shows a 60% decrease in the amount taken out by social security (FICA-OASDI). Net result is a larger take home amount.
Does anyone know why that would happen?
If your annual salary exceeded the maximum amount subject to Social Security tax for 2013 ($113,700 as per JoeTaxpayer's comment), then it is possible that the last paycheck is what put you over the limit. Thus, part of the salary on the last paycheck had Social Security tax withheld and part did not. As JoeTaxpayer points out, there is no limit on the earnings subject to Medicare tax, and so that amount should not have changed at all.