Storing contact list by any service should be illegal. You might try to not share your data, but if anyone has you on their contact list, it’s out of your hands
I wonder if Google has a backdoor API which lets group people by backup email. If one always creates an account with the previous email as a backup, the link is east to make.
That would be especially useful for censorship services across competitors: Although Google Facebook and Twitter compete, sharing the flagged accounts would allow recognizing the same user coming back with a different email address.