That seems incorrectly taken statement. They represent it as though people are leaving social media. That is a false logic. E.g. Peeps may have swapped instagram to snapchat...
It would be interesting to know what % dont use social media by age group.. Something here: http://www.pewinternet.org/2018/03/01/social-media-use-in-20...
I would consider reddit, missing in the article, more of a social network than WhatsApp.
That are just features of every communication platform, even the traditional postal services.
AFAIK, most people consider social media platforms to be a subset of communication platforms with many to many communication, discovery, community etc..
I'm with you though that the groups feature of WhatsApp could be seen as some basic form of social media. (For the sake of theoretic precision; certainly not for introducing censorship.)
The definition of a social network is not in its problems but in its features.
And I'd say one of the defining features of a social network is discovery (e.g. new people, groups that share a common interest, etc.). This is completely lacking in WhatsApp - as you already need to be able to "have access" to the people you want to talk with before you can contact them - but is the integral experience with facebook or instagram.
You can post status updates (images, message, videos, ...) with WhatsApp: https://faq.whatsapp.com/en/android/26000031/
That doesn't mean people are exactly choosing freely now, they might be following the herd because all their friends are following the herd. But at least upstarts can compete for the future herds.