Game is back up!
This is not a drill. The game is down. We’ll let you know when it’s back up. Your refresh game is strong. Go and read something on the Hub while it’s down – that’s what we’re doing at the moment.
Niantic has acknowledged the login issues on both platforms and both games:
We’re currently investigating reports that Trainers are encountering login issues. Thanks for your patience.
— Pokémon GO (@PokemonGoApp) February 15, 2018