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.
I’ve always done all of that via Facebook messenger or discord not via text, before that it was via MSN messenger.
I suppose it makes sense to use iMessage for it, but so few people I know even have iPhones so that’s likely why we never used it.