Social security is the United States' federal mandatory pension, and it does not generally apply to foreign income (unless in a country that also has a mandatory pension with which the US has a tax treaty).
Oddly enough, you can opt out of the Social Security - some employees of government agencies don't have to contribute and some religions are also exempt.
Personally, I feel that anybody should be able to permanently opt-out if desired.
Personally, I feel that anybody should be able to permanently opt-out if desired.