The article is just not doing a very good job of identifying social networks. Whatsapp is BIG. There are also many non-US base social network, like WeChat, that was not on their radar screen.

EDIT: I should explain myself more.

For my mom, her social network is a paper address book and her phone. That's the technology she use to get in touch with her friends.

Whatapps has the necessary technology and feature. What makes it a "social network" in this context is the adoption. If a large number of your peers are using it as a main way to get in touch, it is their "social network". You may not think of it as such because it looks different than Facebook. But this will become more apparent if you are aware how much penetration it has in some communities.