In general, the low IM app usage just means that mobile phone plans are cheap. It's not like the US where you have to take a payday loan to send more than five SMS a month.
I don't buy this. I've had unlimited SMS messages included for as many years as I can remember but pretty much exclusively use IMs. The media integration and things like that are just much better.
That's a recent thing. I remember spending a semester in the US a decade ago. My cheap student ass was surprised to see how expensive every mobile plan was. It was literally cheaper to buy a roaming plan in my own country for six months than to buy a plan in the US.
Just FYI, here I pay 10€ for unlimited calls, text, and data, and 25GB roaming abroad per month.
Sometimes it's better to just say nothing.