It is very convenient to be able to find immediately the accounts of your friends in a social network when you register.
The secure way to do this is generally to store an hash of the email addresses of the user’s contacts.
Couldn’t we imagine (or maybe it already exist ?) a protocol like WebFinger which takes an email hash and return the corresponding account in the instance (if it exists) ? This way, friends discovery could work even accross decentralized networks !