I suspect a fairly small percentage of users is active enough that you get usable hourly data.
Related question, does WhatsApp send the heartbeat only when I open the app, or every x minutes as long as it has a network connection?
It's certainly a more popular app outside of the USA. They initially gained traction because they were willing to make apps for things other than iphones and androids - which gave them a huge following in the developing world where people may still use 10+ year old candy bars.
https://www.nytimes.com/2014/10/30/fashion/tinder-the-fast-g...
But yes not every 2 minutes
SMS took off faster in Europe than in the US, but we've had bundled packages for so long that the individual cost per text wasn't such an issue, and now on many plans they're unlimited.
I guess the differing cost structure depending on who you're texting and from where may have spurred the adoption of WhatsApp, whereas in the US, even if you WERE paying per text, it was the same across a territory of many thousands of miles and hundreds of millions of people. And, the same way that many folks in the US do not even have a passport, they tend also not to have a reason to text internationally. The size and homogeneity of the country benefits the adoptions of some technologies, but hinders the adoption of others.
I think the reason is that a typical cell phone plan in Europe was like 5€ per month, plus 0.07 cents per text (or call minute). Whereas typical American plan was $50 month, but unlimited free text and calls. So people who text lot didn't want to pay even for the tiny amounts for individual text messages, and migrated to using apps.
The all-inclusive fixed price monthly plans are only now getting more popular in Europe.