In the friends list each is tagged with “Today”, “Yesterday”, or “2+ days ago”… What activities trigger a change to that field – logging into the game? opening a gift from a friend? sending a gift to a friend? opening a gift from YOU? sending a gift to YOU? Something else?
Knowing how to interpret that could make it useful, while not knowing how to interpret it renders it meaningless. (It might as well be the number of days since that player last used the restroom…)