I'd love to but me missus would kill me
Are you after writing to someone whose serving away? Whilst not inundated with these sort of letters, there's generally a few floating around each squadron/company in theatre, and anyone who wants to can write back to strike up a conversation. They are much appreciated and a source of morale, and I know of a couple of strong friendships (and one romance) that started like this. You used to be able to just address a letter to 'A Marine' or whatever, c/o a BFPO in theatre. I believe now all letters of that type have to be addressed to someone specific, so you may be best off trying somewhere like forcespenpals, or I think the Sun run a similar scheme. Have a Google. There's not too many actually serving on here, and those that are don't generally pop in so much whilst they're away! Incidentally, it may help to give an idea of your age, as a 20yr old sprog probably won't have much in common with an 'older' woman, and vice versa.