UPDATE: Beeper, through their Twitter account, is saying that they’re working on it and “hope to have good news to share soon”:
Work continues to fix the issue causing the Beeper Mini outage. We know how hard this has been for those who loved using Beeper Mini, and we’re extremely sorry for the inconvenience. We are feeling good, though, and hope to have good news to share soon.
My friends said they had a button pop up to instead send as SMS yesterday. Beeper sent notifications the service was down and shortly after my number was deregistered with imessage without me having to do anything.