Well, since you have unlimited texting, it won't cost anything. I use facebook mobile and the price is the same as regular unlimited texting. Facebook doesn't charge you to use the mobile app, so i'm pretty sure you're fine.
I never had facebook mobile, but I really wanna say if you had unlimited texting it wouldn't cost anything. cuz AIM was the same way, it's use was counted towards text messages so if you had unlimited text it didn't cost anything.
