With this change, you will also be able to link your Trainer profile to your Facebook and Google accounts and use any of those credentials to access Pokémon Go.
The developer explained the request for Facebook access was due to many players losing access to Google accounts, and in turn, the chance to play the game with their existing trainer ID.
To link your Trainer profile to a Google or Facebook account, access the Settings menu and then select the platform you’d like to link.
A large number of players originally signed up for Pokémon Go with Google accounts tied to universities and jobs, which they reportedly lost access to after leaving the school or work position.