mero ni kaatiraa chha. when i asked the payroll office they told me that starting the 5th year in the U.S., we (F-1s) are considered "Resident Alien" and are required to pay SS and Medicare taxes.
but i've some friends who have been in the U.S. for longer than me and they don't pay a dime towards SS and medicare tax. don't know how the system works, man.
let me know if you find out a way to not pay these taxes and/or get the money back for the amount that was taken from your paycheck. i too, am sick of paying 1/5th of my gross income for taxes

for detailed information:
http://www.irs.gov/Individuals/International-Taxpayers/Foreign-Student-Liability-for-Social-Security-and-Medicare-Taxes