People aren’t using Instagram for photos, WhatsApp for text, Line for stickers... they’re using everything for everything.
Seems to be dominated by the younger audience. I look at my younger cousins and see their behavior with the apps. They post and interact with many different social networks. Not my cup of tea as much, but I use Twitter for most of my social network interaction, I post photos on both Instagram and Facebook so the family can see what's going on. I use Linked In for business.
Is FB going to buy Whatsapp, Snapchat, Line, Kakao and the next ten that emerge as well? Sure, some of those will disappear, but it doesn't look like FB will crush the competitors the way it did on the desktop. On mobile, FB will be just one of many.
I think this is the interesting thing about mobile also. It is much harder to dominate like the desktop. Moving between apps is so much easier than going back and forth between websites. I think this is why the Chromebook is a flawed strategy.