| The exemption from social security tax applies only while you are in F-1 status. It does not apply when you are H1-B. See the following from the IRS website:
F-visas, J-visas, M-visas, Q-visas. Nonresident alien students, scholars, professors, teachers, trainees, researchers, and other aliens temporarily present in the United States in F-1,J-1,M-1, or Q-1/Q-2 nonimmigrant status are exempt from Social Security / Medicare Taxes on wages paid to them for services performed within the United States as long as such services are allowed by the United States Citizenship and Immigration Services (USCIS) for these nonimmigrant statuses, and such services are performed to carry out the purposes for which they were admitted into the United States.
Exempt Employment includes:
On-campus student employment up to 20 hours a week (40 hrs during summer vacations)
Off-campus student employment allowed by USCIS
Practical Training student employment on or off campus
On-campus employment as professor, teacher or researcher
Limitations on exemption:
The exemption does not apply to spouses and children in F-2, J-2, M-2, or Q-3 nonimmigrant status.
The exemption does not apply to employment not allowed by USCIS or to employment not closely connected to the purpose for which they were admitted into the United States.
The exemption does not apply to nonimmigrants in F-1,J-1,M-1,or Q-1/Q-2 status who change nonimmigrant status to a status which is not exempt or to a special protected status. The exemption does not apply to nonimmigrants in F-1,J-1,M-1, or Q-1/Q-2 status who become resident aliens for tax purposes. | 
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