I think it would be beneficial to incorporate Whos Online with a Private Message System and Friends / Following.
When a user who is marked as Friend / Following comes online, their Avatar could have a different colored background, maybe even get a notification that they are online.
Clicking on the user (better yet, hover mouse over user to display options) would give options to visit their profile, follow, friend, send a PM, show details of when user last logged in or how long they have been logged in for.
Admins / Mods might see more options like ban or edit profile.
As the forum users increase, it might get to the point where an infinity scroll is used or give the user the option to ignore users or white list users, so only the users you want to know about are shown.