Bingo!
A FAC alone does not have a chance against a submarine since usually most FAC's do not have hull mounted or towed sonars. Some FAC's have TT's (such as HN Combattante II's) but efficiency is very questionnable (obsolete SST-4's)
So, FAC's will require ASW support from other assests such as ASW helicopters (Vesta datalink) and frigates (Link). This leads us to:
1. FAC's do not operate alone
2. FAC's require operating areas around which friendly assests (preferably frigates, ASW aircraft and helo's) are present.
#2 implies that enemy aircraft / helicopters thinking of attacking a FAC must first get rid of those (FAC's) friendly assests. i.e. An F-16 which is going to fire an AGM-84 Harpoon will first have to face the SAM systems of a nearby frigate. This does not make sense to me. If I were the pilot of that F-16, I would fire my Harpoon right into that frigate, instead of the FAC.
So, my point is (and I better come to it, otherwise this will turn something into a strategy palaver
) :
FAC's need defense against ASM's, thus requiring CIWS / PDMS. But since they have to operate in coordination with friendly surface and air assests, they do not need SAM's in first place. Speed and size are their primary defensive weapons against aircraft, ASM's and torpedoes.
RAM is a state-of-art close in defensive system. It's a short range (some 10km in nominal conditions) system because of it's mission definition. Using it as a SAM system is meaningless, since either enemy aircraft will not find the chance to get in closer to 10km because of friendly assets in the region or they will already have fired their ASM's (or other types of weapons).
A FAC having a RAM does not mean much, since the targets which RAM is designed to neutralize are mostly directed to bigger ships, i.e corvettes, frigates, support ships, amphibious ships etc.
It's been a hellish day at the office, so excuse if my message is complicated