Shout messages do little to bring in the kind of member you expect to stay longer then a month.

The best chance you will ever have of getting more members is to socialize and be extra helpful, if you spot a green leaf or fc less guy then ask him if he wants help with anything, craft him some gear or hand him some gil or something.

Then you can send him a friends request and tell him about your fc and if he needs help or wants to join he can contact you, i can guarantee you that those that join you thru this means is more likely to stay longer since you have shown them that your a good person.