How can I make the Facebook app forget everything?
The reason this happens is that deleting app and data deletes only the actual app and the data files it left in its sandbox. This leaves out the information the app has stored in the iOS Keychain. This information is not deleted when an app is deleted.
There could be many reasons why Apple chose to do it this way. For example a large company could have several different apps in a group relying on the same Keychain data - for example Facebook also makes apps like Creator Studio, Portal and Facebook Gaming. If deleting one app deleted the Keychain data as well, the other apps might stop working contrary to what a user might expect.
In an earlier beta release of iOS, Apple did actually experiment with deleting the Keychain data when deleting an app - trying to preserve Keychain data only when other apps in the same group were installed. This lead to confusion for some users as to when the Keychain data was actually deleted - as well as frustration for some users that were used to deleting apps to &quot;save space temporarily&quot; and relied on reinstalling them also making them automatically login again. The change was reverted and never went into the actual iOS release.
As a user there's unfortunately no general way to delete the Keychain data of a specific app. You can only wipe the whole device to delete everything by opening Settings, tap General, then Reset and then Erase All Content and Settings. Beware that this will delete everything on the devices - not just Facebook or the Keychain.
Remember to take care if you're syncing your Keychain across multiple devices using iCloud.