When I first signed up to have Facebook mobile enabled, so I could send status updates and receive Facebook communications via text message, I got hit with two $9.99 premium messaging charges, and started receiving a joke-a-day text that I didn't need or want. Turns out that the confirmation code I had to send to enable the FB texting somehow signed me up for these other services (it all happened at the same time, on the same day) - so although it appeared I opted in, and I did send a confirmation text, it was all done with me unaware of the premium services. Sneaky - but I caught it and was able to cancel and get a refund for the charges.
And I still can text updates and receive from FB - that's just simple domestic texting and is part of my unlimited texting plan. It's the added premium services that piggy backed on that sign up that was sneaky and carried the charges.