Is there any data that shows people in the US are switching to WhatsApp? The only people I've ever seen use it are people with family in other countries. The statistics I've seen indicate that iPhone usage amongst American teenagers is high and still increasing(1), which almost certainly would lead to higher iMessage usage.
how do imessage and android users communicate with each others? Do android users really still use sms to reach apple users? Don't they have group chats everywhere?
Here in europe every club/association/group has a whatsapp group chat. For instance here since the official app provided by the government has a super clunky UX most people get information from primary school through a whatsapp group chat managed by the parent's representative who has exclusive access to teaching group.