This was discussed the previous time this topic came up [0, 1, 2]. This is basically OS level integration which enabled users to share Facebook updates without opening Facebook like OS level integration. Apple removed the Facebook, Twitter, Flickr etc. integration in iOS11 [3].

[0] https://news.ycombinator.com/item?id=17223926

[1] https://news.ycombinator.com/item?id=17229301

[2] https://news.ycombinator.com/item?id=17224071

[3] https://www.cultofmac.com/485346/ios-11-ditches-facebook-twi...