Check this out directly from the Social Security Adminstration...
There is more interesting information at this link.Question: What is the dollar amount of payrolltaxes paid by unauthorized workers and theiremployers for the latest tax year?
Response: We estimate $13 billion in OASDI payroll taxes from unauthorized immigrant workersand their employers in 2010. This number reflectsearnings for those with no recorded SSN, thosewho have obtained an SSN with fraudulent identification, and those with legitimate SSNs who haveoverstayed temporary visas.
https://www.ssa.gov/oact/NOTES/pdf_notes/note151.pdf