FS is a written short form of Foreign Service: the people a country sends abroad to speak for it — diplomats in embassies and consulates — and the office at home that looks after that work. In the United States, Foreign Service is almost a career name: you join the Foreign Service and may live for years in other countries. You meet FS in job ads, CVs, and government papers more than in spoken chat; people usually say the full words. Though actually, the same two letters mean many other things in other worlds (a computer’s file system, full screen, and so on). In this use it is always about diplomacy, not computers.