Facebook hasn't been the best for a long time. It's easy to get lost behind rosy quarterly numbers and miss the forest for the trees. The platform has been on a decline and it's starting to show in its numbers.
There's a difference between decline and lack of growth. Facebook, at this point, has most of the internet connected population of the world signed up. That's a huge deal, and it means that even if Facebook MAU/DAU don't grow, they're in a good place to grow revenue, just by being the de facto gatekeeper for advertising going forward.
Facebook isn’t declining much. More so growth has just stalled. It’s still a huge company and Instagram will continue growing. If they can get Facebook Messenger and/or Whatsapp monetized to even a few bucks per user, that’ll add a decent chunk of change too.