Some people I know seems to be more split between Snapchat, Instagram and Facebook Messenger. I don't see anyone being 99% in one app, and certainly not WhatsApp. I think I know like three people who use WhatsApp privately.

I do know people working in sales and purchasing and I wouldn't be surprised if they had 25% or more of their professional lives in WhatsApp. Previously they used Skype heavily. So this could be interesting for them.