Made the User model 'omniauthable' with 'facebook' as the omniauth_provider

app/models/user.rb

@@ -2,5 +2,6 @@ class User < ActiveRecord::Base
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable and :omniauthable
   devise :database_authenticatable, :registerable,
-         :recoverable, :rememberable, :trackable, :validatable
+        :recoverable, :rememberable, :trackable, :validatable
+  devise :omniauthable
 end