Update 4/14/2019: Trainers, Facebook is under maintenance, and some Pokemon Go players are not able to login into the game via Facebook.
The app is not down globally which is why some region is able to access Facebook and some may not. NN servers are located in US FB servers which are affected by the outage.
Previous Story 3/13/2019: Trainers, Facebook is down at the moment, so players who are using Facebook to log into Pokemon Go will not be able to do so.
At the moment, Facebook is down and site and app are not working for some users. These users are seeing the following message:
Facebook will be back soon. Facebook is down for required maintenance right now, but you should be able to get back on within a few minutes. In the meantime, read more about why you’re seeing this message. Thanks for your patience as we improve the site.
Also, some users are able to get online, but some functions are not working for them – they can’t share photos, can’t update their status etc.
Niantic is also aware of this issue:
If you currently log into Pokémon GO or Ingress using Facebook, this login method is currently unavailable. We're closely monitoring this issue and will provide an update. In the meantime, you can still access both games via alternate login methods. Thanks for your patience.
— Niantic Support (@NianticHelp) March 13, 2019
Now, this is not Niantic’s fault and they are not the ones who can fix this problem. This is Facebook’s problem and they have a global blackout at the moment.
Some players are furios because of the recent event “Special Lunch Hour” and not being able to participate due to the login issues. Niantic is also aware of this and they will be extending for those affected.