The Tax Relief, Unemployment Insurance Reauthorization, and Job Creation Act of 2010, which reduced the social security withholding tax rate paid by employees from 6.2% to 4.2% for wages paid in 2011 and 2012, expired on December 31, 2012. The tax rate for employees will return to 6.2% for wages paid in 2013 with an increase to the wage base limit to $113,700.
Employees do not need to do anything regarding this tax change. It will become effective with the first paycheck of 2013.
Please refer to the following link for more information: http://www.irs.gov/uac/Latest-News