Not the person you're responding to, but aren't there service plans with unlimited texting and images? I'd definitely use this service if I could text.
I'd be fine paying for it to cover the cost of SMS. Like parent, I'm not willing to use facebook to get access, even for free.
One option would be to re-enable the SMS option for people who deposit money to cover the SMS fees. Mark them up 100% (so 2c per SMS or something) and you'll have a built-in profit margin.
Maybe another SMS service offers that? If so, I'd certainly consider SMS again.