Unfortunately, some of us just can't "delete everything." I know it's a mailbox management issue and how much storage space will be enough; but the small amount this forum gives compare to others is ridiculous. And yes, there are already mechanism to "spy", as I said that it's just a convenient. Using PM to check if someone is online is like playing ping pong over long distances. And just because the "spying" mechanism is already there doesn't mean that everyone wants/uses it. I've been searching a way to turn it off or disabled it.
Having indication (green circle) for online around a user id of his/her post is also another way, this of course only works only if you see the user post. Again this is a convenient.
Having a "chat" capability as an alternative communication is nice. Consider when someone mailbox is full, how the hell you're going to tell them if you can't sent PM? There is no indication that's full until you enter your mailbox and check if it's full. Yea, posting telling them to clean it is NOT a solution to the problem, it's a solution to a symptom.