Where did Twitter and Facebook accounts go in iOS 11?
Apple has removed these logins for Twitter and Facebook. From https://www.axios.com/apple-removes-facebook-and-twitter-integration-from-ios-11-2433996734.html:


  The bottom line: Now Facebook and Twitter will be treated like other apps, with the social networks forced to rely on the same iOS sharing extensions used by all those with whom they compete.


Apps relying on this central login logic need to provide an update for iOS11.