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.
And while smartphones recently took lead in sales, there is still huge installed base of dumbphones.
Conversion from dumbphones to smartphones began couple of years ago, and will take many another 5-7 years, until smartphones will dominate installed base.
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.