How does Instagram find my Facebook account? So I deleted Facebook app and messenger and also removed Facebook from the list of accounts in settings. I also cleared Instagram data and deleted the app itself. 

What is puzzling me is that when I reinstall Instagram it prompts me to login with my old Facebook account. 

How does it know about this account? Where are the remnants of it being stored? 

Backup appears to be off too
Apps that use the newer Android standard of data backup will store their basic information on Google's servers... 

Look in Settings - Backup &amp; Reset and there is an option called Automatic Restore (or something similar), it's description literally says "When reinstalling an app, restore backed up settings and data". 

If you do not want the applications data restored, remove the app, turn this option off and reinstall the applications.