You say that now, but Twitter also began as a way to send a quick message to your friends.
You used to be able to use it as a way to create a free self-managing SMS distribution list. Pretty cool, before I had phone with data capabilities.
That was even a use case I understood. Now I feel too old to understand how to use twitter. But that can't be it. Much older people seem to have no problem.
But, it wasn't built to scale like WhatsApp.
Umm it quickly pivoted to being a social networking site. Before they even were trying to scale they were a social network, micro blogging site.