It is huge in Central Europe at least. Pretty much everyone is on it, you would be left out of a lot of functions if you didn't have access to it (eg my kids childcare has its parents' chat groups there, my building block has a group there. In online dating, asking for a number to move the discussion to Whatsapp is pretty common... Heck, last time I brought my bike into the repair shop, they asked me if it was okay if they sent me the status report via Whatsapp).
To be fair though, while Whatsapp is the default messenger for pretty much everyone, both signal and telegram also have their users, especially with younger people. Eg the volunteers for helping the Ukrainian refugees in my city organize via telegram, and the above-mentioned bikeshop ended up messaging me via Signal.
That makes sense why it's passed me by then. I do find it fascinating how different systems still take hold in different places even in the age of the global web tying us all together. Just from following the war in Ukraine and seeing how tightly the Slavic world has grabbed on to Telegram was enlightening too.
It is huge in the US also. The main feature Whatsapp enabled was free international messaging. So it immediately blew up in immigrant US populations and bigger cities where there would be more international connections.
At the time Whatsapp came out, I was in NYC, and I do not recall anyone not using WhatsApp. I remember how amazing it was to effortlessly chat with my UK family since we were no longer subject to super expensive international SMS/MMS charges.
I would consider Viber up there, if not more popular with some immigrant communities. I've never used either, but it was interesting to see the adoption of it essentially driven by mobile users in other countries with family members in US--as opposed to the other way around.
To be fair though, while Whatsapp is the default messenger for pretty much everyone, both signal and telegram also have their users, especially with younger people. Eg the volunteers for helping the Ukrainian refugees in my city organize via telegram, and the above-mentioned bikeshop ended up messaging me via Signal.