I have this problem too. It's not simply a matter of the FB carrier setting, as I have discovered that my phone is sending "STOP ALL" every morning at about 6am. It is definitely sending it to Twitter (40404), GameStop (46787), and lots of different facebook shortcodes (32665100, 32665103, 32665175, etc., possibly all 100 of the codes facebook uses). I know this because I get a confirmation from gamestop that they will stop sending me texts, my twitter status becomes "STOP ALL", and I get dozens of replies from facebook shortcodes saying "Sorry this message cannot be processed on this short code." (Probably those FB short codes which I haven't used recently.) The texts are probably being sent to other shortcodes as well, I have no way of knowing because no outgoing texts appear in my phone's messaging app. Multiple virus scanning programs have reported my phone as completely clean. I have seen a couple of posts in other forums blaming it on a Yahoo account sync, with users claiming that the problem went away after removing Yahoo, but I don't have any Yahoo content on my phone at all.
I am running a Samsung Galaxy Prevail on CDMA with Boost's Unlimited with Shrinkage plan. I just switched to this phone a few days ago.