I'm happy that we got what you where looking for into Track. The release notes for 13.1.6600 did mention this since that is the release it came out in.
Changed: Top navigation icons now show and hide automatically, depending on the available space.
I can see how this note could be read to not fully convey what the feature does.
At this point we have no plans to add an counter to the Who's On icon. This is partly to maintain consistency with the other icons that also do not have counters. Also depending on the sites traffic theses numbers would constantly fluctuating.