SS is a pay-as-you-go system. Currently, SS is running an annual surplus, but it will start declining in 2008 and will go into the red in 2017. For those illegals who are paying into SS, they are helping the size of the surplus, albeit in a minimal way. The SS surplus is put into the General Fund to be spent on anything Congress wants. The amount of the "surplus" is deposited into SS Trust Fund in the form of non-market T-Bills, i.e.,IOUs. The SS Trust Fund contains no real assets, but rather, liabilities, which is why the SSTF is included in the $9 trillion national debt [as intragovernmental holdings,] which consumes 17 cents out of every dollar to service annually.
So, one could say that the illegals' contributions to SS actually increases the amount of the "surplus," which adds to the size of our national debt. If that money were not available from the illegals, the money would have to be borrowed or spending reduced. Again, the illegals are not a major factor in SS collections.