The optional Who's online block on the sidebar lists all currently logged in users. The information can emphasize social aspects of a site. It also can be invasive. And for a production site, if the list is very short it is a bit embarrassing.
... we don't expect massive traffic, but worth a test and during development can be useful ...
During one of our weekly phone calls, the team was skeptical that this was useful. It also can be a bit invasive. So, this feature has been disabled at the site. It can always be re-enabled if needed.