Kind of besides the point, but Facebook Messenger, Snapchat and iMessage has very good penetration, people usually have two of the three. And everyone just fall back on SMS when if they need to.
Do they really use Snapchat for writing? I've tried to use it besides picture sharing and it's really cumbersome...
It's really slow to startup (compared to other apps), the UI is confusing, and old messages get deleted...