Crush is for whatever people want it to mean - for some people it's a 'proper' crush (and I know several people who first met their partners online, and had a relationship that way before ever meeting in real life, some of whom are now in long term relationships) and for some it's just a handy term for the person on your flist who makes your heart leap everytime you see their icon and realise that you get to read a new post or comment.