A subscriber's 'Last activity' timestamp is updated when the user:

• opened a campaign or autoresponder
• clicked a link in a campaign or autoresponder
• subscribed to a list
• unsubscribed from a list
• bounced
• marked as spam
• confirmed a double opt-in subscription
Subscribers are sorted by 'Last activity' in the list. Therefore, subscribers who did any of the above activity will float to the top of the subscriber list. You will also see changes in the 'Subscribers activity chart'.
I really want to see this feature in to the dashboard because this will help in list cleaning, it will be easier to identify inactive users and remove them.

Screen shot attached.

Thank You.