The way that you are tracked by Threads is when you click on a link (to load a list of posts in a forum, for example). Because the chat page is static, and Who's Online updates every 10 minutes, people have a tendancy to "fall off" of who's online when they are in the chat.
JoshPet solved this problem by creating a java script that updates your online status every 9 minutes, but I don't use it in my hack.
The odd thing that I've noticed is that sometimes I'll show up on a different page in the who's online display, even though the only page I have open is the chat. Unfortunately I don't know enough about how the who's online script works and I haven't had time to sit down and figure it out.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.