"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. For example, someone who currently logs in with Pokémon Trainer Club could link their Trainer profile to both their Google and Facebook accounts, and then they can use whichever account they prefer going forward."
Originally shared by Pokémon GO
Good news, Trainers! We’re rolling out account linking with Facebook: https://pokemongolive.com/post/accountlinking/