devise.rb

config.omniauth :facebook, ENV["FACEBOOK_APP_ID"], ENV["FACEBOOK_APP_SECRET"], callback_url: "https://supersalad.herokuapp.com/users/auth/facebook/callback", scope: 'email', info_fields: 'email'

User.rb

def self.from_omniauth(auth)
where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
    user.email = auth.info.email
    user.password = Devise.friendly_token[0,20]
    #user.name = auth.info.name   # assuming the user model has a name
    #user.image = auth.info.image # assuming the user model has an image
  end
end

omniauthcallbackcontroller

    class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    # You need to implement the method below in your model (e.g. app/models/user.rb)
    @user = User.from_omniauth(request.env["omniauth.auth"])

    if @user.persisted?
      sign_in_and_redirect @user, :event => :authentication #this will throw if @user is not activated
      set_flash_message(:notice, :success, :kind => "Facebook") if is_navigational_format?
    else
      session["devise.facebook_data"] = request.env["omniauth.auth"]
      redirect_to new_user_registration_url
    end
  end

  def failure
    redirect_to root_path
  end
  end

logs

2016-12-15T02:56:29.416929+00:00 heroku[router]: at=info method=GET path="/users/auth/facebook" host=supersalad.herokuapp.com request_id=ed6b9aa3-320d-4987-82d6-048d76d6ccf9 fwd="24.188.181.153" dyno=web.1 connect=1ms service=7ms status=302 bytes=2406
2016-12-15T02:56:29.407760+00:00 app[web.1]: I, [2016-12-15T02:56:29.407684 #4]  INFO -- : [ed6b9aa3-320d-4987-82d6-048d76d6ccf9] Started GET "/users/auth/facebook" for 24.188.181.153 at 2016-12-15 02:56:29 +0000
2016-12-15T02:56:29.408014+00:00 app[web.1]: I, [2016-12-15T02:56:29.407971 #4]  INFO -- omniauth: (facebook) Request phase initiated.
2016-12-15T02:56:29.644312+00:00 app[web.1]: I, [2016-12-15T02:56:29.644230 #4]  INFO -- : [07c1fa4b-07f7-4f91-bfdc-48295a4485fa] Started GET "/users/auth/facebook" for 24.188.181.153 at 2016-12-15 02:56:29 +0000
2016-12-15T02:56:29.644604+00:00 app[web.1]: I, [2016-12-15T02:56:29.644553 #4]  INFO -- omniauth: (facebook) Request phase initiated.
2016-12-15T02:56:29.650149+00:00 heroku[router]: at=info method=GET path="/users/auth/facebook" host=supersalad.herokuapp.com request_id=07c1fa4b-07f7-4f91-bfdc-48295a4485fa fwd="24.188.181.153" dyno=web.1 connect=1ms service=4ms status=302 bytes=2406
2016-12-15T02:56:29.834463+00:00 app[web.1]: I, [2016-12-15T02:56:29.834423 #4]  INFO -- omniauth: (facebook) Callback phase initiated.
2016-12-15T02:56:29.834238+00:00 app[web.1]: I, [2016-12-15T02:56:29.834169 #4]  INFO -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6] Started GET "/users/auth/facebook/callback?code=AQD6pOPo__KVGK9x6CVw-UeswvhsgjK8QznwpQpLND4qKGFXARY6YOMkKr6Tbanw9Ppi7cGwU4nlBkndwlfc9AB-daRhzzCzi-hXcIGNmoqzXHXYvbNXMGzt4IzHWB_qVk2xBmaiqjnZNeC8YatfsDD29iuhOcj1Ki4VmAgwM9kfR_UttXTRe_2Ew3NK8yXp2RB-Z6WSGfFXARjTrEGXGste-eU5UOjDWAL99LgjwDrKGUupcA7AJQ4QG-tz8VCtU_U_a2lmTG1oDeCJKa6IbTxNYhyB_Tq429faf-wx1t8SXamFVstDc2a0G0eSyEqRyoU&state=92ea0ef94a8ea98a2b9fb719710a04317d3107368792b7c5" for 24.188.181.153 at 2016-12-15 02:56:29 +0000
2016-12-15T02:56:29.927251+00:00 app[web.1]: I, [2016-12-15T02:56:29.927208 #4]  INFO -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6]   Parameters: {"code"=>"AQD6pOPo__KVGK9x6CVw-UeswvhsgjK8QznwpQpLND4qKGFXARY6YOMkKr6Tbanw9Ppi7cGwU4nlBkndwlfc9AB-daRhzzCzi-hXcIGNmoqzXHXYvbNXMGzt4IzHWB_qVk2xBmaiqjnZNeC8YatfsDD29iuhOcj1Ki4VmAgwM9kfR_UttXTRe_2Ew3NK8yXp2RB-Z6WSGfFXARjTrEGXGste-eU5UOjDWAL99LgjwDrKGUupcA7AJQ4QG-tz8VCtU_U_a2lmTG1oDeCJKa6IbTxNYhyB_Tq429faf-wx1t8SXamFVstDc2a0G0eSyEqRyoU", "state"=>"92ea0ef94a8ea98a2b9fb719710a04317d3107368792b7c5"}
2016-12-15T02:56:29.927207+00:00 app[web.1]: I, [2016-12-15T02:56:29.927124 #4]  INFO -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6] Processing by Users::OmniauthCallbacksController#facebook as HTML
2016-12-15T02:56:29.933406+00:00 app[web.1]: D, [2016-12-15T02:56:29.933352 #4] DEBUG -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6]   User Load (1.6ms)  SELECT  "users".* FROM "users" WHERE "users"."provider" = $1 AND "users"."uid" = $2 ORDER BY "users"."id" ASC LIMIT $3  [["provider", "facebook"], ["uid", "10154682770005962"], ["LIMIT", 1]]
2016-12-15T02:56:30.083929+00:00 app[web.1]: D, [2016-12-15T02:56:30.083836 #4] DEBUG -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6]    (0.6ms)  BEGIN
2016-12-15T02:56:30.098408+00:00 heroku[router]: at=info method=GET path="/users/auth/facebook/callback?code=AQD6pOPo__KVGK9x6CVw-UeswvhsgjK8QznwpQpLND4qKGFXARY6YOMkKr6Tbanw9Ppi7cGwU4nlBkndwlfc9AB-daRhzzCzi-hXcIGNmoqzXHXYvbNXMGzt4IzHWB_qVk2xBmaiqjnZNeC8YatfsDD29iuhOcj1Ki4VmAgwM9kfR_UttXTRe_2Ew3NK8yXp2RB-Z6WSGfFXARjTrEGXGste-eU5UOjDWAL99LgjwDrKGUupcA7AJQ4QG-tz8VCtU_U_a2lmTG1oDeCJKa6IbTxNYhyB_Tq429faf-wx1t8SXamFVstDc2a0G0eSyEqRyoU&state=92ea0ef94a8ea98a2b9fb719710a04317d3107368792b7c5" host=supersalad.herokuapp.com request_id=deeaa598-287c-4cd3-8e38-d1b9ffa11ed6 fwd="24.188.181.153" dyno=web.1 connect=1ms service=262ms status=302 bytes=1927
2016-12-15T02:56:30.089487+00:00 app[web.1]: D, [2016-12-15T02:56:30.089397 #4] DEBUG -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6]   User Exists (0.8ms)  SELECT  1 AS one FROM "users" WHERE "users"."email" = $1 LIMIT $2  [["email", "yourmom12369@gmail.com"], ["LIMIT", 1]]
2016-12-15T02:56:30.092020+00:00 app[web.1]: D, [2016-12-15T02:56:30.091958 #4] DEBUG -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6]    (0.5ms)  ROLLBACK
2016-12-15T02:56:30.092973+00:00 app[web.1]: I, [2016-12-15T02:56:30.092906 #4]  INFO -- : [deeaa598-287c-4cd3-8e38-d1b9ffa11ed6] Redirected to https://supersalad.herokuapp.com/users/sign_up