The rates are specifically set. Both the employee and the employer pays a share, which has not always been an equal share, nor are the amounts fixed year from year.
Additionally, The Social Security Component has had a wage limit, and earnings above this are no longer taxed. In 2013, an additional Medicare tax went into effect for high wage earners (no portion of this tax is paid by the employer as of this writing). Always confirm the tax rates and wage limits for the year in question.