When I was shopping for a mobile phone plan for my sister four years ago unlimited SMS were hard to find. Sure, now you get them everywhere, but I don’t think standard smartphone contracts had them four years ago. You had to pay obscenely more (something like double the monthly cost) if you could get unlimited SMS at all. I remember that the high cost of SMS were a real sticking point for my sister.
In my experience smartphone contracts in Germany tended to be much cheaper than in the US but you would also get substantially less for your money. That’s still the case today with included unthrottled data volume.
So maybe that’s one reason why Whatsapp took off. Another one I can think of is that Germany is less of an iOS monoculture than the US. Android is more widespread, so iMessage frequently gets you no benefit at all.
Yes, that is exactly what it means unless there's some research data provided. Obviously it is easier to scale our current surroundings to the whole country, than to really go around and see how things are outside our social circle. Then again, same applies to you too, just because you haven't heard of WhatsApp before doesn't mean it wouldnt be big in the UK. :)
I lived in the US for a while, and I can't really come up with any reason why it should be different there. Only that way more people are on Facebook (and those are also way more active). Still, I can't really explain why texts stuck around even for smartphone users.