Hey man
My recommendation is to just talk to her when necessary. Even turn your chat window on Facebook off so it appears that you are offline. I'd recommend only talking to her every now and again but you definitely do not want to be messaging her on a daily basis. If you messaged her on a daily basis it would appear that you don't really do much with your life and girls really do not like this.
Just message her if you have something to ask/tell her. But do not feel the need to message her just for the sake of it.
It will also be a lot easier to talk to her extensively in person if you haven't talked to her online for a while.
Good luck!
