I hope users who would like to see bulk messages on their phone chose to opt-in (manually, not auto-opt-in). Your terms of use do not demand this.
In the US: "FCC rules ban text messages sent to a mobile phone using an autodialer unless you previously gave consent to receive the message"
I would wonder deeply about opt-in rates as well. How are you keeping track of how much users enjoy receiving these messages? (Not just click rates because people might click something they hate to receive)
Also, confusingly, your terms of use state:
> You agree that You will not use the Service to send.. "promotional materials"...
But that's what's shown as an example on the front page?