Maybe its finished
But there has to be somewhere for the people leaving to go. After getting used to being able to connect with their friends in this way, most people aren't simply going to give it up.
They obviously aren't going to Twitter, they are already in decline and the stock price is showing it. Ditto for Snapchat. I see some friends whose only "posts" on Facebook are sharing photos via Instagram, so maybe people are going there?
Maybe it isn't just one place, and there are people going to a number of different places, but if anything is subject to network effects it is social networking, so a diaspora across a half dozen different platforms seems unlikely - if Facebook experiences a downturn people will end up going somewhere else in similar numbers, eventually.
One thing I have noticed is that my business page gets less and less "organic reach" as time goes on. They want people to pay and are reducing the reach deliberately, no doubt. As businesses see it is no longer useful to be on Facebook unless they pay ever escalating fees to get the same reach they once got for free, they'll find alternatives as well.
Some may retrench on the web, some may have a custom app developed (you can get branded "custom" apps for a given class of business like restaurant, retail, etc. pretty cheaply) and encourage your customers to download the app by making discounts or promotions available through without having Facebook act as a toll collecting middleman.