Total registered users: the number of people who have ever set up an account on a grid. InWorldz now has 63,929.
Active monthly users: the number of people who have logged into the grid at least once over the past 30 days: InWorldz now has 6,621.
Currently logged in: users who are, right now, logged into the grid. That's the number you see in your viewer, usually around a couple of hundred.