They've maintained their positions through acquisitions. Facebook itself could be on a slow decline, but if Instagram and Whatsapp do well are they really dying?
What I would be worried about is that the same practices of monetization that killed Facebook are seeping into Instagram. At what point are they just a poison that slowly kills anything it touches? I think they already are but the deaths are slow.
I think they realize this though. Just milking the cow now. Anyone smart enough realizes social media itself is relatively new and volatile. They have to be fluid / versatile in their investments and services other wise people just simply move to the latest and greatest platform. That's what happened in the early days with Myspace / Facebook. A business built upon this kind of product is a business built upon sand. It moves and shifts. There is no one size fits all. At least I don't think so anyways.
This makes me really sad that this is the goal of Facebook, who will then serve only the goal of capitalizing on the latest and greatest type of platform format (by acquisition or copying) for social media while systematically and methodically milking the patience of their users with ads.