[rovek]

I'd be interested to know what drives these differences, whether it can be measured, and why there seems to be limited crossover of "circles".

In the UK, I've had WhatsApp for about 12 years since the Nokia N900. If someone gives me their number, it's assumed we will communicate by WhatsApp.

As an interesting crossover, my American friends communicate with their friends/family in the US with iMessage but use WhatsApp for everything else and the only Brit I've met who used iMessage by default had spent significant time in the US in recent years.