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.