My mind was blown when Blackberry allowed WhatsApp on their devices. BBM was so sticky to so many people that if they hadn't had WhatsApp to transition to other devices to, I think Blackberry would still be alive and kicking today.
We did start to hit push service quotas. They quota by the app, not per user with the app. So we had a quota of 5 million pushes per day. Asked for more. Denied. We found some creative alternatives. PIN messaging was a surprisingly open API. So we ended up having currently connected devices send a PIN message to devices we want to bring online. The recipient device would delete the message to cut down on noise.
Somebody senior in Engineering actually responded, to their credit, but the response was basically "because security, which is why we're the #1 platform".
Aged like milk.