New feature is here - private messages! Now you can send private message to a friend by starting message with @username, e.g. if you have friend called "John", send sms
In sms, it would look like 5d, Vita: @YOU: Molodca! Ja s toboj!
First, there are 5.6B dumbphone users in the world, vs. 835million smartphone users. And take into account, that large share of smartphone users do not use mobile internet, or use WiFi only. This means, that there are roughly 10 times more people who can send/receive sms vs. those who can browse mobile web or use apps.
So, if you want a social network that could be used TODAY by all of your friends, by people that either do not care about mobile fashion, or older people, or poorer people - sms is the way to go.