There is a cost associated with staying connected to friends or colleagues should circumstances change (e.g. change of address or job). Twenty years ago, if a friend moved to a different city, the options for maintaining that connection were letters and phone calls. Even with the emergence of communication technologies like email, IM, VoIP etc sharply cutting communication charges, the ongoing cost of tracking identifiers and addresses remained. When costs are high, limited time and energy will ensure that not all connections can be maintained - they get lost in the tail.
Applications that facilitate the maintenance of such connections by lowering the associated cost (e.g. in time, effort, duplication across various applications, etc) could be useful. A connection that might otherwise get lost in the long tail of the social distribution could keep its head above the water.
I have been thinking that something like Liberty's People Service could play a role here, imagining that somebody could build an application on top to help people stay in touch with those from which they would otherwise drift apart. Call it 'Virtual Mom' maybe. Seemed a bit far-fetched.
Then I came across this paper describing a 'Keep in Touch' Phone - a phone that reminds/persuades users to do just that, keep in touch with their contacts.