With the iOS 11 update on iPhone and iPad, social network services, like Facebook, Twitter, Vimeo, and Flickr, which used to have single sign-on for systemwide integration have been removed. Instead, these services are treated the same as other apps. You'll have to give permission within apps using the standard extensions that other apps use.
In iOS 10 and older (back to iOS 8), users could sign into Facebook, Twitter, Vimeo, and Flickr from the Settings app on iPhone and iPad. This would make it easier to add additional content, like contacts, events, and more, directly to Apple's built-in apps, like Contacts, Calendar, Photos, and others. Additionally, apps that requested access to those social services could integrate with just a quick permission from you.
In iOS 11, Facebook, Twitter, Vimeo, and Flickr are treated the same as other apps. App permissions, contacts, calendar events, and other content will have to be added via the Share extension.
This also means that Facebook and Twitter integration with Siri has changed. You can no longer send a tweet with Siri's help or update your Facebook status through Siri on iPhone and iPad.
With Siri Search and Use with Siri you do have some integrated features. Siri can search Facebook and Twitter, and when Use with Siri is enabled, you can do things like asking Siri to look up the Facebook contact for someone. But, no more asking Siri to update your status or send a tweet.