Nothing. I mean probably she saw it by opening the message container but not the chat window
so you shouldn't do anything abouut it except if you know her from before besides, screw the facebook bro, it sucks because you have no interaction and you can't build attraction and even if you are talking to her for the first time on FB it's hard to make her interested because she probably get's messaged by random guys on daily basis....and i've figured it out it's a easy way to get friendzoned...hope you understand what i mean 
Peace