title: Facebook dialog, logout and access token (Facebook iOS SDK)
body: <p>When a Facebook dialog is created with code like this</p>
  
  <pre><code>NSMutableDictionary* params = [NSMutableDictionary dictionaryWithObjectsAndKeys: apiKey, @"api_key", nil];
  [facebook dialog:@"stream.publish" andParams:params andDelegate:self];
  </code></pre>
  
  <p>is there the possibility to use sessions?</p>
  
  <p>I succeeded in publishing a post on a user's post wall, but how allow the user to logout?
  Moreover I obtain this error when I try to access to current user information.</p>
  
  <pre><code>{
      error = {
          message = "An active access token must be used to query information about the current user.";
          type = OAuthException;
      };
  }
  </code></pre>