Starting with 2011 it was $106,800 The tax is payable on the first $106,800 of earnings. Earning are defined slightly differently for this than what is used for withholding, (or other things)....
Especially as at @110K a year it covers the overwhelming majority of workers, it just may not add that much to the coffers. More than that, the higher earners through life pay much more in and don't...