They take out 7.65% of your gross pay on the first $112,500 (for 2013) and then 1.45% on everything above that -- no matter how you are paid.
Oh, and this is taken off the top. However, YOU ARE STILL TAXED (via the income tax) on the amount they take out. So, not only do you NEVER see it, you're taxed on top of that on the money you NEVER see. Cute, isn't it.
This post was edited on 1/4 at 1:45 pm