Social Security benefits are paid according to a progressive formula that gives low-wage workers a better rate of return on their contributions than it gives high-wage workers. The progressivity of the benefit formula outweighs the disproportionate burden imposed by the tax. As a result, low-wage workers can expect to receive benefits that exceed the sum of their and their employers' payroll tax contributions. Middle- and high-wage workers, on the other hand, can expect to pay substantially more into the system than they will receive in benefits. Thus the Social Security system redistributes income from middle- and high-wage workers to low-wage workers. In addition, many low-wage workers recoup some or all of their payroll tax contributions in the short run through the Earned Income Tax Credit (EITC). Overall, middle- and high-wage workers subsidize the income and payroll tax liabilities of low-wage workers, leaving most low-wage workers with net negative tax liabilities throughout their lifetimes...
Social Security tax revenue is distributed among three trust funds which finance different tiers of the Social Security system: the old-age and survivors insurance program (OASI), the disability insurance program (DI), and Medicare Part A hospital insurance (HI). The total OASDI portion of the FICA tax is equal to 12.4 percent of the first $65,400 of wages and the HI tax is equal to 2.9 percent of total wages. The tax is shared by employees and their employers with each paying half of the total percentage. Employers can deduct their share of the contribution for income tax purposes, but workers' shares are not tax deductible
Source:
http://www.house.gov/jec/fiscal/tx-grwth/payroll/payroll.htm