FICA taxes are separate from the income taxes you compute on your return. You really are being taxed less.
You still don't get it. Yes FICA is a separate category. However under the current tax code you owe a certain amount of FICA taxes at the end of the year. Unless they change the rate: (which they have not by the way) it is NOT a TAX decrease.
At the end of the year when you or your accountant are doing your taxes you will still owe the same amount for Federal and FICA. If they didn't take out enough, guess what? You will have to PAY, or you will get a smaller refund.