I thought "promo codes" referred here to the things you stuck at the end of URLs that took you to the app's install page—sort of like Amazon affiliate IDs. I find the idea that you'd be actively telling people your code, rather than e.g. typing their email address into the app to send them an "invite" link with the code embedded, kind of weird and unintuitive.
At this point, emoji support is almost ubiquitous (at least in the US). I'd imagine that within the next couple of years emoji support will be within a rounding error of 100%.
There's no obvious way to type emojis on Windows 10. Also, most emojis are almost unintelligible at small sizes because of the black and white font and faulty ClearType rendering.
Great point. Typing emojis on non-mobile devices is a pain right now. Cmd+Ctrl+Space on Macs brings up an emoji-picker, but I doubt many people know that.
I guess this should be scoped to mobile-only applications. Web services can also use traditional referral links so a memorable referral code isn't as big of a UX win.
I think the idea is bad overall. Old versions of Android, still widespread, can't enter emoji either.
Best referral code I've seen is a phone number: that way you know it's not going to spread beyond the inner circle of family and friends of the referrer. (Unless you don't want that, of course.)