Since you reside in South Africa for more than 183 days each year, and I presume you have been doing this for more than three years, you are considered a SA resident subject to income tax on your earnings. If less than three years you are considered a temporary resident and not subject to SA tax on income earned elsewhere. You will be eligible for a foreign tax credit in the US for income tax paid to another country. Social Security and Medicare are not affected by your residency.