IM has massive deployments in big corporations. Part of that is that Lync/Skype is kind of a throw-in with Office/Exchange setups these days, but there are millions and millions of seats with IM clients running and being used.

We've got customers that are doing tens and hundreds of thousands of chats per day across their organizations.

I've seen Lync (Skype For Business) gain some traction in more corporate settings.

I remember using an IM client linked to lotus notes (was it sametime?) when I worked in a corporate setting 10 years ago.