Why do you think it is inevitable that people would use WhatsApp or Instagram if they decide to leave Facebook? Is that really as obvious as you are making it out to be? Is there any evidence of that?
The point is that finally, FINALLY the mainstream is paying attention to the potential ramifications of having a consolidated identity on social media, and how they potentially could have their perception of reality distorted by using such services. Now is the time to embrace changes if they come, not to get complacent at the news cycle based on ones own personal cognitive biases.