It is not a decline, it is just "maturation" or even "over-ripeness". Speaking for a decline, it is rather a decline of interest in "internets" in general and

It is not a decline, it is just "maturation" or even "over-ripeness". Speaking for a decline, it is rather a decline of interest in "internets" in general and especially social networks in particular. It is no longer a "shiny new thing". So, Google, as FB, are trying to squeeze everything what is left from G+ and its flagship Gmail, just because everything switches to mobile and a chat is a new email. That is why they are pushing Hangouts and FB pushes Messenger, which both wants to hijack your SMS app. But chat apps are too simple and it is not so easy to push ads here, because users will just switch to less annoying rival's service. So, let's say that it is a decline of browser-based "internets", and email as a default way of communication, not just Google or FB.