I'm not sure but there might be a max.
Each year, a contributions base or FICA limit applies for Social Security taxes. In 2012, the contributions base or FICA limit is $110,100. If you earn more than that amount, you get a break. Once you have paid taxes on $110,100, your employer takes no Social Security taxes from your paycheck until the next calendar year. Even if you are retired and collect Social Security benefits, you pay FICA taxes on your earned income up to the $110,100 limit.
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http://www.ehow.com/info_8389626_fica-limit.html