Simple enough, when looking at a friend on your list, be able to see the last time they were on. Could be a little calendar (or clock) icon (similar to the friend finder icon) and hovering over it tells the last date they were on. Could say "Less than a day ago" when it's under 24 hours, otherwise give the date. That way if the player is online but 'hidden from friends', it wouldn't reveal that they are on in some way, but would let the friend know that they were on recently.
This would help greatly when going through a friends list trying to decide who can be removed to make room for a new friend.
