Don't forget you can also send messages to the regular inbox, so no need to be running the app at the same time, which would, as you said correctly not exactly

Don't forget you can also send messages to the regular inbox, so no need to be running the app at the same time, which would, as you said correctly not exactly a way to replace SMS. As to the penetration rate, the people that I communicate 95% of the time are all on facebook. Maybe I need to add some older people to my social circle :D