As another datapoint, iMessage is probably the least useful thing to me in the whole ecosystem. Apple has nowhere enough market penetration around here (N Euro) for it to be useful as anything more than an SMS app.
I live in the US but have many friends from northern Europe and I've noticed the same thing. The only thing I use WhatsApp for is communicating with my European friends, and every single one of them prefers it over text or iMessage even though they all have iPhones.