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`iPhone only?': `No'

What else is there? You only have an App Store link on the site.

What we mean by this is that recipients do not need to have an iPhone (or even the app) to be able to participate.

How is that different than iMessage, which also seamlessly falls back to SMS for anybody that doesn't have iMessage support?

With iMessage, those on SMS (who don't have an iPhone) won't see everyone else's replies. You can't do group chat over SMS.