Sometimes you're talking to someone, right? You send them a message and their last 'activity' on their page is messaging you before that. So you think they're still online, and keep messging and messaging them to stop ignoring you. (Just an example, no one would really do this lol. I think.)
But what you don't know is that they had to go to lunch or something.
What I'm suggesting is actually pretty simple- just add something that says they're online somewhere on their profile, or let us click '__ guests' at the bottom of the page