Yeah I've heard that argument before, but I don't believe it holds weight. Nor do I feel the metrics are in lockstep with the story that people try to tell. So again, I'll ask for a source please. Lets face it, whatsapp and instagram are complimentary to the service facebook provides as a social network.
Instagram is no more like facebook than twitter is. Instagram could have been the next facebook, but FB made a great defensive play. Snapchat is fighting versus whatsapp, kids post instagram pictures and then link them to facebook, and whatsapp is an OTT instant messenger. FB is still a completely viable social network and really the only one in it's class. It's like arguing craigslist is going to fail because AirBNB went after one of their silos. FB is more than posting pictures, it's more than messaging, it's more than picture messaging. It has 1.1 billion users and is basically fully saturated in America.