Using a fake social security number is not as clear cut it seems. If a fake, or phony SS # is used then SS taxes will be taken out and put into the SS system. However, that person will never be able to collect that money. And I don't think they care.
Many times these people use the same, and sometimes a valid, SS #. Again, the taxes will be deducted and put into the system. I'm not sure if the actual owner of the SS # will be able to collect all that $$ as I believe the SS admin has computer programs that validates money accredited to any one account. If Sam Gonzalez shows he paid SS taxes while working 25 jobs at the same time an investigation just might kick off.
Another note on many people using the same SS #: this happens all the time, and not just with illegals. Many times it's the paper pushers who have to fill out forms or punch data into a system who enter a dupe SS # when the guy in front of them can't remember his.
So, I'm sure illegals do pay some taxes but, 25%? I'd like to see that data.