I use Signal and WhatsApp daily. Most people I know use some sort of IM, though for some of them it's only iMessage.
It's interesting and a little surprising that none of the PC-based instant messaging services popular in the early 2000s were successful in making the jump to mobile, though some did produce mobile clients.
Facebook messenger was quite something through. They bundled messenging into the Facebook app to get the network effect, then use the network effect to get people to install a separate messenger app.
IMHO, the reason a mobile first network was more compelling than a desktop first network is that people tend to have their phones on them, while their desktops tend to be at their desks. For better or worse, you can always reach me on my phone.
Tencent developed QQ on desktop and then WeChat on mobile, but afaik, didn't bridge the two networks.
I've read that it comes down to how at the time of the shift, companies were realizing that there was just no money in running IM networks, so they were unwilling to invest much in them.