They could take the phone numbers, sort them and hash them together. So if Sam is 5 and Jack is 6, they both upload the hashed social relationship 56 to the system and it can match them up.
It wouldn't keep someone with access from checking if a social relationship existed in the database, but it should make recovering phone numbers and the like from the hashes quite a lot harder.
That only works though if you have both Jack & Sam's phone books (and they have each other in their books) so the hit rate would go down, possibly significantly.
It wouldn't keep someone with access from checking if a social relationship existed in the database, but it should make recovering phone numbers and the like from the hashes quite a lot harder.