Most people on facebook are not on facebook. They have inactive profiles. They may check to peep those who are active, but beyond that, there is very little utility or upside to those who quit caring. And this is always a simple function of time; everyone quits caring eventually. Facebook will continue to insist these peepers are "active" but no, this bluff was tried by Google+ and it won't fool anyone. Those looking for a job might clean up their profile just in case, but this doesn't mean they're on or using facebook.
Facebook will become the next myspace. That's why Facebook, being run by people who know this well, is buying what could be to facebook what facebook was to myspace. That's why Instagram and WhatsApp needed to be purchased.
Facebook is moving beyond a platform. Social media to them is now about real estate. You can move off from facebook to instragram like one would from Santa Monica to Venice. But your landlord is still facebook.
Here is one concrete example of why Instagram is amazing and Facebook sucks. When a brand posts something on Instagram, there is no "promoting" their post, there is no "mining impressions", and there is no "paying for likes". There is no machine learning optimized feed. Instagram pushes a photo to everyone instantly, and the response is also unencumbered and immediate. And it has no ads. Unlike facebook, Instagram does not stand in between you and your followers. All their efforts into the quality of what facebook should be doing on facebook, yet the answer was to not be there at all. The presence of the "host" is not welcome in any social setting, not online or offline. We don't need the waiter or waitress to feed us at the restaurant while reading ads. That's facebook.
Seriously, facebook sucks. It's future is dead. Even just for the reason that my mom has twice as many friends than I do and all her peers love it. She just turned 70.