On the desktop version of Nabble see the green dots on a user's avatar that indicate a user is online.
Perhaps you are viewing on a mobile device where some less important features of the desktop screen are not displayed. such as folder and pinned topic icons and user's avatars. The avatar is required to display the green dot.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.