Currently we don't tell contacts that we've removed them - we just stop connecting to them and close any connections they make to us, since we no longer recognise the tags.

The main advantage of the current approach is that we can remove contacts tactfully: the contact can't necessarily tell whether we removed her or whether we just haven't signed in recently. However, if the contact sees us posting to forums, blogs or private groups, she may be able to tell that we've removed her. A second advantage is that we can immediately delete all state relating to the contact. Removing all identifiable state is important - it's the equivalent of forward secrecy for the social graph. But removing all state is just convenient.

The main disadvantage of the current approach is that the contact wastes battery and bandwidth trying to connect to us indefinitely. Depending on the transport this may expose metadata (#62). These problems will get worse over time as users accumulate defunct contacts.