I don't mean to advertise, however, if anyone is reading this and is worried, then I highly recommend a (FREE) Android app called TextSecure. I can't say for certain that it is out of the FB app's "scope," if you will. But what I do know is that if two people have TextSecure, their SMS messages can be encrypted using OTR*. What I also know is that many "spy" apps and "SMS backup" apps cannot monitor TextSecure messages (even unencrypted messages.)
This app is free and should be available in the Play market, and the APK can be found online but I cannot vouch for the source if you get it from any shady site. I encourage you to research similar apps if you are worried about mobile security. Any questions about anything please feel free to PM me.
* - OTR is short for Off The Record. It's a plugin for many chat programs such as Adium for OSX, and Pidgin for Windows/Linux. It encrypts text conversations using a very efficient method which can detect Man-In-The-Middle attacks and other vulnerabilities. It's touted that the conversations also cannot be replicated even if someone were to have the key. I don't know the scientific details, but encourage you to research more if you are interested.
Phenom X6 1090T, XFX Radeon 695x 2GB, 16GB Corsair RAM, Coolermaster, Fatal1ty 750W PSU, Gigabyte GA-990FXA-UD3. Yeah, I keep it on a budget.