@jospoortvliet @bert_hubert @niels @Karlitschek What I do know: How to turn it off:
Admin -> Sharing -> 'Federated Cloud Sharing' -> 'Allow people to publish their data to a global and public address book'
Which "mildly" understates the "sends for all users" part. -.-'
Brb, playing with my cats, feeding them, and then figuring out who all gets a data leak notification from me. -.-'