It is usually used by women but maybe a man could use it
with his girlfriend. For a woman it should only be used
with people she is very close to. Close friends, or a
boyfriend.
Some people will sign their letters;
xoxo (hugs and kisses)
xxx ooo (hugs and kisses)
xxx (kisses)
Well, some of my male chatfriends use it quite often to me during our chats even though they have a girlfriend. I think it denpends on a person in what kind of situation you use it. I use it only to my BF, maybe because I am a Japanese and I'm not used to use it to other people.