Today (Nov 12, 2012), we released a change to People Analytics API. Specifically, responses from the /api/2.0/engage and endpoint now includes a virtual $last_seen property.
The $last_seen property reflects the last time a people record was updated. We are only tracking updates that have happened since today so many of your people records will not have a $last_seen set. $last_seen will automatically be updated whenever you update a user's people record.