What exactly is declining? Their MAU, Time Spent, # Advertisers, and Revenue numbers are all increasing (https://techcrunch.com/2017/06/27/facebook-2-billion-users/). Maybe they're just declining in eyes of SV bubble residents.
And # of Advertisers and Revenue are proportional to how people think they're doing, not how they're actually doing, and would presumably only decline after a mass exodus.
(and your linked article is 9 months old talking about even older data so is not really relevant to the current state of affairs.)