And Google+'s Hangouts. And Skype's IMs. And tweeting to each other can also be seen as chatting.

I'd say the biggest chance is the fact that all of them are web-centric or mobile-centric.

Twitter has DMs, Snapchat has text chat, Skype has IM, Google+ has hangouts, Instagram has direct messaging.

What about Skype?

And if we are counting Facebook's Chat, most people I know uses Talk (Google+ Hangouts).

Some of my friends do in fact use SnapChat for its chatting capabilities.