Germany, The Netherlands, Switzerland, etc. WhatsApp dominates all of these markets.
I think a reason for this is that in Europe, we're not used to unlimited text plans. Unlike the US, we don't have to pay to receive text messages, only to send them, so there is little incentive to get an "unlimited plan" anyways. But ultimately, you still end up paying 20 cents or so for a text message, whereas with WhatsApp you pay at max 1 buck per year.
Secondly, for a long time, WhatsApp's user experience was simply much better than what Facebook had to offer with their Facebook messenger app. There simply was no alternative to WhatsApp for quite some time.
I actually resisted hopping on the WhatsApp train for a long time, but when virtually everyone of your friends uses it, you will join up sooner or later.