There should be some text in a user's profile where it tells people when this user was last active, this would be useful for the user's friends to determine whether he/she is still active.
You could.....or you could say whether or not you are active in your About Page
Being-a-jerkery out of the way, we used to have a last active feature (in text form I believe), but this caused EXTREME lag. Like, even worse than the other features here. So the administration axed it and here we are.
But really, using it on your bio is fine. It won't take too much out of your day
