How to logout completely from Facebook using Ruby on Rails and Devise + Omniauth. I'm just modifying the Omniauth Railscast http://railscasts.com/episodes/236-omniauth-part-2

application.html.erb

<div id="user_nav">
     <% if user_signed_in? %>
          <img src="<%= user_avatar %>" id="main_avatar"> Signed in as <%= current_user.email %>.<br />
          Not you? 
		
          <% if session[:fb_token].nil? %>
               <%= link_to "Sign out", destroy_user_session_path %>
          <% else %>
               <%= link_to "Sign out", facebook_logout_path %>
          <% end %>
     <% else %>
          <%= link_to "Sign In", new_user_session_path %>
          <%= link_to "Sign Up", new_user_registration_path %>
     <% end %>
</div>

application_controller.rb

  def facebook_logout
    split_token = session[:fb_token].split("|")
    fb_api_key = split_token[0]
    fb_session_key = split_token[1]
    redirect_to "http://www.facebook.com/logout.php?api_key=#{fb_api_key}&session_key=#{fb_session_key}&confirm=1&next=#{destroy_user_session_url}";
  end

authentications_controller.rb

class Users::AuthenticationsController < BaseController
  layout false

  def create
    omniauth = request.env["omniauth.auth"]
    session[:fb_token] = omniauth["credentials"]["token"] if omniauth['provider'] == 'facebook'

    # ... Same as Railscast ... #

  end
  
  def failure
    render :text => "Login Failure!"
  end
  
end

routes.rb

  match '/auth/facebook/logout' => 'application#facebook_logout', :as => :facebook_logout
  match '/auth/:provider/callback' => 'users/authentications#create'
  match '/auth/failure' => 'users/authentications#failure'

  devise_for :users, 
             :controllers => {:registrations => 'users/registrations', :sessions => 'users/sessions'}

sessions_controller.rb

class Users::SessionsController < Devise::SessionsController
  
  def destroy
    super
    session[:fb_token] = nil
  end
  
end

A simple way to logout from facebook from your app's server side is to redirect it to facebook.com/logout
In logout method, add
redirect_to "https://www.facebook.com/logout.php?next=#{redirection_url}&access_token=#{fb_token}"
where, redirection url, is the url where facebook would redirect/callback after logout. For example, "http://www.example.com:3000/thankyou"
fb_token, is the token you get from auth_hash['credentials']['token'] , where auth_hash = request.env['omniauth.auth']

PS: omniauth-facebook gem is being used here (https://github.com/mkdynamic/omniauth-facebook)

Thanks
Devyani@livvel